Guided-wave optics method is used to determine the temperature of the crosslinking polymer films. In the parameters of polymer films are determined according as the guided modes peaks in the attenuated total reflection spectrum depend strongly on the refractive index of polymer films. The refractive index change of the crosslinkable polyurethane is investigated with the ascending temperature. Experimental results show that the curve for the change of refradive index with temperature dn/dT evolues uncontinously, and there exist two intersection points. The temperature range between the two intersection points is regarded as the polymeric crosslinking temperature range. The first and second intersection point are regarded as the beginning temperature of crosslinking and the completing temperature of crosslinking, respectively. The crosslinking temperature range of the polyurethane, namely optimal poling temperature range, is from 85 ℃ to 122 ℃. The measured result is examined and convinced by the measurement of electro-optic coefficient of the poled polyurethane film with several temperatures in the range of crosslinking temperature at which the poling process starts.