To effectively monitor the insulation state of the optic-electric composite submarine cable, the IEC60287 cable′ ampacity and temperature calculation standard were improved by iteration algorithm and the temperature field model of 110 kV YJQ41×300 mm2 submarine cable was established with the finite element analysis, solver and simulation software package COMSOL based on the thermoelectric coupling model. The results were validated by the improved IEC60287 and the optimal model size of the submarine cable at different values of ampacity was obtained. The effects of ampacity and ambient temperature on the optical fiber′s temperature rise were investigated in normal insulation state and at different levels of insulation degradation of the submarine cable. Dielectric loss factor was used to characterize the insulation states, the change of optical fiber′s relative temperature rise under the typical value of dielectric loss factor was investigated. The results reveal that, under normal circumstances, the temperature rise of fiber is approximately proportional to the square of ampacity and it increases approximately linearly with increasing ambient temperature with a slow velocity. When the cable′s insulation degraded, the relative temperature rise of the optical fiber is proportional to the tanδ. Comparatively, the ambient temperature has little influence on it. According to above research, a formula for estimation of dielectric loss factor based on relative temperature rise and ampacity of cable is presented and a method for insulation degradation evaluation of the submarine cable based on relative temperature rise of optical fiber is proposed. Compared with the existing insulation monitoring methods based on electrical measurands, the error caused by electromagnetic interference can be avoided in proposed method, which can be applied in the online monitoring of the optic-electric composite submarine cable.