Abstract: Biogas slurry can be applied to straw return in sustainable agriculture. In this study, full straw return with biogas slurry was used to reduce chemical fertilizer for high soil fertility. 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). A comparison was made of crop yield, yield components, soil aggregate stability, straw decomposition rate, and soil nutrient contents after the four treatments. The results demonstrated that biogas slurry was the key factor in substantially reducing chemical fertilizer while maintaining stable crop yields under full straw return. The wheat and rapeseed yields showed no significant difference under a 50% chemical fertilizer reduction with full straw return and biogas slurry (W-T4 for wheat rotation, and R-T4 for rapeseed rotation), compared with the control treatment (W-T1/R-T1), which received half straw return and full chemical fertilizer. Notably, the highest wheat yield of 8162.3 kg/hm² was recorded in W-T4 treatments. Biogas slurry alleviated the initial growth inhibition of crops caused by full straw return and improved yield components. In wheat, the spike length significantly increased by 17.4% under W-T4 (10.8 cm), compared with W-T3 (9.2 cm). In rapeseed, the thousand kernel weight was reduced significantly in R-T4 treatment, whereas the number of pods per plant increased. Specifically, pods per plant reached 360.7 in R-T4, which was 33.5% higher than that in R-T3. The combined application of biogas slurry and straw return rapidly improved soil structure, superior to that of straw return alone. W-T4 and R-T4 treatments 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 was 32.0% higher under W-T4 than that under W-T3, indicating the substantial improvement in soil physical stability and resistance to erosion. Furthermore, the integrated practice simultaneously elevated overall soil fertility, although biogas slurry application failed to significantly promote the decomposition rate of rice straw (i.e., no acceleration effect was observed compared with straw return alone). 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 indicated that biogas slurry contributed short term nutrient supply and longer term soil fertility. In summary, the full straw return with biogas slurry reduced chemical fertilizer use by 50% with high crop yield, simultaneously improving soil structure and nutrient status. Multiple synergistic mechanisms also included nutrient supplementation from biogas slurry, physiological compensation for early season growth suppression caused by straw incorporation, and physical amelioration of soil aggregation. The finding can provide promising technical support for green agricultural production, thus contributing to both chemical input reduction and soil quality enhancement in rice rotation systems.