There are many interfering factors in infrared water detecting in tunnel, hence the detecting accuracy would be affected. The mechanism of infrared water detecting used in site test of entrance section of Xianghe Tunnel is analyzed. It is revealed that lamp and the shotcrete have significant impact on infrared water detecting results; the primary cause of the interference to infrared water detecting lies on environment temperature variation induced by the heat generated by interference sources. The main methods to prevention interference, i.e. carrying out detecting work with minimum interference source, interference temperature field establishment and detecting right along tunnel axial line and some angle between receiving hole and detecting lines, are discussed.