The invention provides a waste heat constant-temperature energy saving and emission reducing device for a fuel oil internal combustion engine, and belongs to the technical field of internal combustion engines. The waste heat constant-temperature energy saving and emission reducing device comprises an air cylinder, an exhaust port air inlet fan, a communication pipe, an oil tank and an air inlet, and further comprises a heat exchange tank, a spiral pipe, an atomizer, a heat transfer table, a heat exchange pipe and an activation tank. An exhaust port of the air cylinder is connected with the heat exchange tank. An outlet of the heat exchange tank is connected with the air inlet through the communication pipe. The heat exchange pipe is arranged on the top of the air cylinder through the heat transfer table. The atomizer is arranged at the inlet end of the heat exchange pipe, the activation tank is arranged at the outlet end of the heat exchange pipe, and the oil tank is arranged at the inlet end of the atomizer. The waste heat constant-temperature energy saving and emission reducing device has the beneficial effects that waste heat of the internal combustion engine is utilized and recycled, mixed oil gas is subjected to preheating treatment, the ignition success rate and the combustion rate can be effectively increased, and energy saving and emission reducing are achieved; and high-frequency vibration of burning explosions can be effectively relieved through uniform atomization and pre-activation, the service life of the internal combustion engine is prolonged, noise pollution is reduced, and the comfort level is improved.