Abstract: To provide a theoretical and practical basis for establishing a water-nitrogen coupling model suitable for rice growth under straw return in the Chengdu Plain, the relationship between water management, straw return, N application, and rice root morphogenesis was studied through field experiments. The effects of water-nitrogen coupling on the rhizosphere environment and root development of hybrid indica rice (Oryza sativa) ‘F You 498’ under wheat straw return conditions were investigated with the application of two water treatments, three N treatments, and two straw return treatments. The two water treatments were alternate dry-wet irrigation (W1) and submerged irrigation (W2); the straw return treatments were full-burying and return of wheat straw (S1) and no straw return (S0); and the N treatments involved the application of 150 kg∙hm⁻² of N applied at ratios of basal∶tillering∶panicle fertilizers of 3∶3∶4 (N1), 7∶3∶0 (N2), and 0 (N0). The results showed that compared with W2, W1 promoted straw decomposition before the jointing stage, increased the total organic acid content of rhizosphere soil after the jointing period (1.38%–8.49%) and microbial biomass C content of rhizosphere soil before maturity (0.25%–12.93%), increased root activity during the entire growth period, and increased root morphological indices of the rice population, except at 10 days after transplanting and at the mature stage (18.53%–75.83% of root length, 10.57%–101.33% of root number, 2.49%–88.24% of root volume, and 8.91%−68.08% of root surface area). Compared with W1, W2 increase the total organic acids (3.34%) and microbial biomass C content (2.69%–6.23%) at the mature stage, and the decomposition of straw and the root morphological indices of single stems (12.03%–27.21% of root length and 9.05%–51.44% of root number). Compared with S0, S1 treatment inhibited root morphology development and reduced root activity (2.47%–45.83%), but increased the total organic acid content (8.02%–22.74%) and microbial biomass C and N levels of rhizosphere soil (1.58–31.22%), and the effect of improvement was highly noticeable under W1. The application of N fertilizer promoted the decomposition of straw, increased the total organic acids and microbial biomass C and N contents of rhizosphere soil, promoted root development, and increased root activity. Compared with N0, the optimal N application mode (N1) promoted the root development of single stems (8.27%–38.09% of root length and 2.96%–36.66% of root number) and promoted and maintained the root activity (2.26%–156.35%) in the middle and late growth stages; whereas the conventional N application mode (N2) increased the population root index (12.68%–44.32% of root volume and 4.91%–55.82% of root surface area) and root activity only in the early growth stage (22.01%–29.31%). W1 irrigation coupled with optimized N application promoted straw decomposition, significantly accelerated root growth and development, increased total organic acid content, microbial biomass C and N contents, and root activity in rhizosphere soil, and delayed root senescence. Therefore, under the condition of straw return, the optimized N application mode with W1 is the most suitable agriculture model.