The piston's thermal field and deformation of air-cooled engine were studied to solve the problem that piston temperature is difficult to measure in high-altitude environment.The temperature field and thermal deformation of piston were calculated by thermal-solid coupling FEM(finite element method);the contact pair and thermal resistance were applied to couple the multipart.The loads of temperature and gas pressure were applied to the piston.The deformation under gas pressure can be helpful to reshape the piston when the piston structure is designed specially;so the conclusion provides the practical reference for further improving the structure and optimizing the design of piston at high altitude.