Contamination is a crucial factor that has great influence on the life time of mechanical cryocoolers. First, the generation of contamination sources is clarified in this paper. Then, the Monte Carlo method and the mass transfer equation are used to simulate the complicated mass transfer process of gas contamination in coolers. Finally, the influence of gas contamination on the performance of coolers is analyzed. Some contamination control procedures and life estimation methods of cryocoolers are proposed.