In this paper, the reasons of window pollution are reported, it is mainly deposition coating from the molybdenum electrode spraying and the copper atom spreading. For a domestic 10 W copper vapor laser, after being operated 100 kWh, we measured that the proportion of molybdenum and copper of the coating was about 3:1 by chemical method and the window transmissivity reduced continually from 80% to 65%. In addition, the influence of window pollution on the output power is discussed. At last, the window pollution is reduced by means of prolonging the tube nearby the anode side, and the improving has been used in practice.