Influence of heat treatment on microstructures and properties of 17-PH alloy by laser solid forming (LSF) process is investigated. Results of optic microscope (OM) and scanning electron microscope (SEM) show that main microstructure of LSF 17-4PH includes quenched martensite, tempered martensite and second precipitated phase. As aging temperature increases, martensite becomes smaller and more uniform. Rate of transformation from quenched martensite to tempered martensite is accelerated. Precipitated phases begin to coarsen, grain boundary becomes distinct. After heat-treatment, alloy plasticity is highly improved, and the strength and hardness reach maximum when aging temperature is 480 ℃. With the aging temperature increasing, strength property and hardness get worse, while plastic property gets better. Analysis on fractography shows fracture of all tensile samples results from ductile fracture. In addition, fiber area and shear lip of fracture become bigger, and size and depth of dimples increase gradually with the aging temperature increasing.