The invention relates to an isothermal compression type heat engine taking air as a thermodynamic working medium. The heat engine is technically characterized in that an isothermal air compressor is employed to replace a heat insulation air compressor, two engineering links, i.e. air compression and expansion working are performed in different engine components separately so as to substantially improve the heat engine working pressure, and a temperature gradient type heat preservation composite pipe is arranged to efficiently recover and utilize the tail gas waste heat, thereby obviously improving the thermal efficiency of the heat engine. When necessary, the isothermal air compressor and a low-temperature refrigeration instrument are used cooperatively to make toxic and harmful gas components in tail gas automatically condense and liquefy under an artificial low temperature environment and make them separated from clean gas, and then the toxic and harmful gas components are collected to serve as chemical raw materials, thereby realizing zero emission of toxic and harmful substances in engine tail gas and zero pollution.