It is found in the experiments on synergy between low hybrid wave (LHW) and electron cyclotron wave (ECW) that the LHW reflection coefficient decreases with the injection of ECW into plasmas in the presence of LHW. In the present paper, the coupling characteristics of LHW, and the pump-out phenomenon caused by ECW are analyzed theoretically. The analysis shows the law of pump-out and the increase of plasma density in the vicinity of the grill mouth caused by pump-out, there by improving LHW coupling. The conclusion is proved by the experiments. Also, considering the change of the plasma density in the vicinity of the grill mouth caused by pump-out in synergy experiments, the synergy current should be corrected.