Abstract: This study aimed to investigate the feasibility of integrating full straw return with biogas slurry application to achieve simultaneous chemical fertilizer reduction and soil fertility enhancement. A field experiment was conducted in rice–wheat (W) and rice–rapeseed (R) rotation systems. Two factors were designed: straw return amount (half return or full return) and fertilizer type (full chemical fertilizer, or biogas slurry substitution with chemical fertilizer reduction). Four treatments were established: T1 (half straw return + full chemical fertilizer), T2 (full straw return + full chemical fertilizer), T3 (half straw return + 50% chemical fertilizer + biogas slurry), and T4 (full straw return + 50% chemical fertilizer + biogas slurry). Crop yield, yield components, soil aggregate stability, straw decomposition rate, and soil nutrient contents were compared across the four treatments. The results demonstrated that biogas slurry was the key factor for achieving substantial chemical fertilizer reduction while maintaining stable crop yields under full straw return conditions. Under a 50% chemical fertilizer reduction combined with full straw return and biogas slurry (W-T4 for wheat rotation, R-T4 for rapeseed rotation), wheat yield and rapeseed yield showed no significant difference compared with the control treatment (W-T1/R-T1), which received half straw return and full chemical fertilizer. Notably, wheat yield in W-T4 reached the highest value among all treatments, recorded as 8162.3 kg·hm^-2. Biogas slurry effectively alleviated the initial growth inhibition of crops caused by full straw return and optimized yield components. For wheat, spike length under W-T4 (10.8 cm) was significantly increased by 17.4% compared with that under W-T3 (9.2 cm). For rapeseed, the R-T4 treatment compensated for a reduction in thousand-kernel weight by significantly increasing the number of pods per plant. Specifically, pods per plant in R-T4 reached 360.7, which was 33.5% higher than that in R-T3. The combined application of biogas slurry and straw return rapidly improved soil structure, and this amelioration effect was superior to that of straw return alone. Treatments receiving biogas slurry (W-T4 and R-T4) significantly increased the proportion of macro-aggregates larger than 2 mm in diameter, as well as the mean weight diameter (MWD) of soil aggregates. The MWD under W-T4 was 36.0% higher than that under W-T3, indicating a substantial improvement in soil physical stability and resistance to erosion. Furthermore, although biogas slurry application did not significantly promote the decomposition rate of rice straw (i.e., no acceleration effect was observed compared with straw return alone), this integrated practice simultaneously elevated overall soil fertility. Soils under the W-T4 treatment exhibited significantly higher contents of total nitrogen, alkali-hydrolyzable nitrogen, and available phosphorus compared with the original soil before the experiment. These nutrient improvements suggest that biogas slurry contributes not only short-term nutrient supply but also longer-term soil fertility building. In summary, the integrated practice of "full straw return combined with biogas slurry application" enables a 50% reduction in chemical fertilizer without compromising crop yield, while simultaneously improving soil structure and nutrient status. The underlying mechanisms include multiple synergistic effects: nutrient supplementation from biogas slurry, physiological compensation for early-season growth suppression caused by straw incorporation, and physical amelioration of soil aggregation. This practice provides a promising technical support for green agricultural production, contributing to both chemical input reduction and soil quality enhancement in rice-based rotation systems.