Abstract: This study proposed a combined orifice-trapezoidal weir to address the engineering limitations of traditional trapezoidal weirs, including excessive upstream backwater height, susceptibility to sediment deposition, and frost heave risks in the channel upstream of the weir plate. To investigate the hydraulic characteristics of the combined orifice-trapezoidal weir under submerged conditions and provide theoretical guidance for practical applications, a 1:1 hydraulic physical model was constructed based on a standard trapezoidal weir with a 500 mm weir plate width. Flow gradients ranging from 27.14 to 64.22 L/s were tested, with bottom orifice heights (z) set at 50, 60, 70, and 80 mm. The hydraulic characteristics under submerged discharge conditions were analyzed, including critical submergence thresholds, transition patterns of submerged flow regimes, longitudinal water surface profiles, and upstream velocity distributions near the orifices. A discharge calculation formula for submerged flow was derived using dimensional analysis and multivariate nonlinear fitting. Experimental results revealed the following hydraulic characteristics: The mean critical submergence thresholds for orifice heights of 50, 60, 70, and 80 mm were 0.665, 0.696, 0.716, and 0.754, with variances of 0.0190, 0.0150, 0.0079, and 0.0078, respectively. The critical submergence thresholds increased with orifice height, while measurement variances decreased. As the orifice height increased, the dominant critical submergence regimes transitioned sequentially from impinging jet, broken wave, surface wave, to surface jet, with stable submerged regimes (surface wave and surface jet) predominating. Compared to conventional trapezoidal weirs, the combined orifice-trapezoidal weir reduced upstream backwater heights by 29.32%, 33.83%, 39.47%, and 42.86% for orifice heights of 50, 60, 70, and 80 mm, respectively, under identical flow rates. Based on water surface profile analysis, upstream water level measurement was recommended at positions over 0.10 m upstream of the weir plate, while downstream measurements were advised at locations 2.00 m or farther downstream. All four orifice configurations effectively prevented sediment deposition upstream of the weir plate under submerged conditions, as flow velocities near the orifices exceeded the non-silting threshold of 0.3 m/s. Comprehensive hydraulic performance analysis indicated that the 80 mm orifice height exhibited superior characteristics. When the opening height z was within the range of 50 to 80 mm (with z treated as a variable), the validation results of the general prediction model indicated that 94.48% of the measured values had relative errors between the measured and predicted values of less than ±5.00%. When z was equal to 80 mm (exhibiting optimal hydraulic characteristics and engineering adaptability), the validation results of the prediction model (with z held constant) demonstrated that 100.00% of the measured values showed relative errors between the measured and predicted values of less than ±5.00%. Compared to traditional standard trapezoidal weirs and improved trapezoidal weirs, the combined orifice-trapezoidal weir demonstrated significant advantages in reducing upstream backwater height, minimizing sediment interference, and preventing the formation of frost heave hazards due to residual water in front of the weir plate. This research provides a reference for hydraulic characteristic studies of similar orifice-weir combined flow measurement facilities, offering a low-interference, highly adaptable, high-precision, and convenient solution for flow measurement in terminal channels of irrigation districts.