The invention relates to a waste heat recovery process method and system for an ammonia synthesis device. In the process method, cold desalted water in a cold desalted water pipeline is divided into two parts, wherein one part is fed in a deaerator for deaeration, hot exhaust gas, generated during the deaeration process, is exhausted from a hot exhaust gas outlet in the top of the deaerator, and is fed in the lower section of an absorber from a hot exhaust gas inlet, and after being pressurized by a pressure regulating valve, the other part is used as an absorption power head to be fed in the absorber to conduct mass transfer and heat transfer with the hot exhaust gas in the lower section of the absorber. Waste heat in the hot exhaust gas is absorbed by the cold desalted water and forms hot desalted water. Non-condensable gas is exhausted from a non-condensable gas outlet in the top of the absorber. The hot desalted water is led from the top of the absorber, is collected by a waste heat recovery tank, and is fed in the deaerator for deaeration from a hot desalted water inlet of the deaerator. The feedwater formed after deaeration is drained from a feedwater outlet. The method and the system have the advantages that the process is simple, control is simple and convenient, the reliability is high, waste heat can be recovered, the noise is reduced, the environment is friendly, and the purposes of saving energy and reducing consumption are achieved.