The relatively low yield strength of austenitic stainless steels limits their structural applications. Therefore, this study aims to fabricate ultrafine-grained austenitic stainless steels with ultrahigh yield strength through deformation annealing and systematically investigate how annealing temperatures regulate microstructural evolution, Cu-rich particle precipitation, and mechanical properties. Using 201LN austenitic stainless steel as the starting material, we performed 60% cold rolling followed by annealing treatments at 600°C and 700°C. Comprehensive microstructural characterization was conducted using OM, TEM, and EBSD, and mechanical properties were evaluated through universal testing. Annealing at 700°C for 1 min and at 600°C for 600 min resulted in ultrafine-grained austenitic stainless steels with an average grain size of approximately 0.8 μm. Notably, highly ordered nanoscale Cu-rich particles were observed for the first time in the sample annealed at 600°C. Mechanical property tests showed that the sample annealed at 700°C for 1 min achieved a yield strength of 786 MPa with 40% elongation. In contrast, the sample annealed at 600°C for 600 min exhibited a considerably higher yield strength of up to 1300 MPa while maintaining 25% elongation. This enhanced strength is attributed to the combined effects of grain refinement and precipitation strengthening induced by high-density coherent Cu-rich particles, with precipitation strengthening contributing approximately 342 MPa to the yield strength. The synergistic effect of grain refinement and precipitation strengthening effectively improves the strength of austenitic stainless steel while retaining favorable ductility.