Abstract: 5CrNiMo alloy steel for HVAC was tested. The effects of different solution temperature on the microstructure and mechanical properties of 5CrNiMo alloy steel were analyzed. The results show that, with the increase of solid solution temperature, austenite continues to dissolve, and enter the ferritin body. The content of austenite in the matrix continues to decrease, the original continuous austenite becomes intermittent structure and forms many independent small size austenite units. In the process of tensile deformation, the stress change of the alloy steel shows the law of first rapid increase, then slow increase and then rapid increase. With the increase of solid solution temperature, the tensile strength and yield strength of the samples decrease, and the elongation increases at first and then decreases.