Abstract: To address the issues of frequency drift and unstable echo signals in traditional chipless radio frequency identification (RFID) tags, which suffering from insufficient radiation gain, poor directional focusing, and environmental humidity interference during the nondestructive detection of quality deterioration in fresh meat. In this study, high-frequency electromagnetic simulation software was employed to design a spiral array tag based on the rotation phase principle. The structural parameters of the spiral array were optimized to enhance electromagnetic coupling among array elements. Through electromagnetic simulation, the resonant characteristics, input impedance matching, and radiation performance of the array tag were systematically analyzed, and cooperative radiation as well as improved directional focusing of the array elements were achieved. At the material level, an MXene/In₂O₃ composite material constructed through a heterojunction synergistic effect was selected as the ammonia-sensitive layer. This composite material effectively combines the high electrical conductivity of MXene with the excellent gas-sensing characteristics of In₂O₃, which is beneficial for enhancing volatile ammonia adsorption capability and stabilizing the electrical response behavior. The prepared composite material was uniformly deposited onto the sensitive region of the spiral array tag through a screen-printing process, thereby successfully fabricating a chipless RFID spiral array ammonia sensor and realizing integrated coupling between the gas-sensitive material and the radio frequency resonant structure. To validate its practical applicability, taking fresh shrimp as the target of quality deterioration detection in fresh agricultural products, a radio-frequency sensor detection system was established. The amplitude response characteristics of the sensor return loss (S₁₁) before and after ammonia adsorption were systematically investigated, and the S₁₁ resonant frequency drift induced by humidity variations was further analyzed. To reduce the influence of humidity interference on the detection performance, the coupling mechanism between humidity-induced S₁₁ amplitude response and frequency response was clarified. On this basis, an S₁₁ amplitude–frequency response separation method was proposed to effectively decouple the humidity-induced resonant frequency drift signal from the ammonia-induced amplitude response signal, thereby achieving humidity interference compensation and improving the reliability of ammonia detection. Simulation and experimental results show that, compared with conventional single-tag structures, the introduction of the spiral array structure leads to a 20.17% decrease in the S₁₁ magnitude of the tag and a 102.33% enhancement in the radiation gain along the Z-axis, which effectively strengthens electromagnetic energy coupling and radiation capability on the tag surface. Within an ammonia concentration range of 0～15 mg/L, the sensor exhibits a good linear response relationship with a sensitivity of 0.65 dB·L/mg. During the testing process, humidity variations cause both S₁₁ resonant frequency drift and amplitude fluctuations. Under 80% relative humidity, the concentration detection error of the sensor is significantly reduced by 91.89% after applying the humidity interference compensation, demonstrating that the proposed signal decoupling strategy effectively improves detection accuracy under high-humidity conditions. Furthermore, the sensor exhibits excellent packaging penetrability and low-temperature adaptability, with its sensing response attenuating by only about 2% under low-temperature conditions. In summary, the proposed chipless RFID spiral array ammonia sensor demonstrates significant advantages in radiation performance enhancement, humidity interference suppression, and detection stability. The results confirm that the array-based structural design combined with the S₁₁ amplitude–frequency decoupling method can effectively improve wireless sensing reliability in complex environments. The proposed sensor can be effectively applied to the wireless nondestructive detection of quality deterioration in fresh meat such as fresh shrimp during cold chain storage and transportation.