Investigation of relationship between the heat caused by electricity in cables and change of the optical unit transmission characteristics is very important for the design and application of fiber composite low voltage cable(OPLC). Stable operation and short circuit fault state of fiber composite low voltage cables are simulated with COMSOL software, and the corresponding cable temperature distribution and optical unit transmission loss characteristics are obtained. The characteristic points of different position on the cable are selected for simulation. Results show that the temperature rising is obvious in the conductor insulation layer when the cable is in trouble, and the outer sheath temperature change is not obvious. The fiber temperature change is very small, only 0.2 °C rising within 5 s. The displacement caused by thermal expansion is very small, so that the transmission loss are almost the same in the two cases. Short circuit faults have little effect on the optical fiber temperature. Temperature rise test of optical unit is designed to obtain optical cable transmission loss data, and those are compared and analyzed with the simulation data. The measured temperature data lag behind the simulation data 5 s, but it is consistent with the change trend of simulation data. It is proved that the simulation model is reliable and feasible.