Due to different influences of cooling rate on the solidification structure of ZL205A alloy under different temperature gradients, the ZL205A alloy with stepped structure of different wall thicknesses was used, and the specimens of different thicknesses were prepared under different solidification conditions. The effects of cooling rate and temperature gradient on solidification structure and tensile strength and elongation of alloys after T5 treatment were explored. In addition, the correlation effect of the phase transformation law with the structure and performance was explored. Results show that the temperature gradient caused by different wall thicknesses have a certain impact on the microstructure and morphology of the alloys. Reducing the temperature gradient can significantly improve the mechanical properties of the alloys. After T5 treatment, the tensile strength of the alloys achieves 506 MPa.