The temperature of power cables in an underground cable conduit, which depend on ambient environment and operational current, can reach levels in excess of the maximum allowed values in summer. Over temperature due to accumulation of heat accelerates the aging of power cable insulation materials, and increase risk of fire accident. In order to limit the operating temperature of cables in an underground cable conduit, an optimization method is proposed in this paper to reallocate the current ratings of the multiple cable circuits in a cable conduit. Based on the Finite Element method, the temperature field in an underground conduit which houses a group of cables is calculated. An optimization model of reallocating the current in the cable group is established. On the basis that the temperature of the highest temperature cable in the cable group should never be allowed to exceed 90°C, and achieves a minimum and that the total current of all the cables in the conduit should remain unchanged, the cable with the highest temperature in the cable group is optimized by distributing its current. It is found from results of simulation by Comsol Multiphysics that, compared with before the results of not adopting the proposed optimisation method, the conductor core temperature of cable with the highest temperature in the conduit is reduced by almost 11% and the maximum temperature difference of among the cables in the conduit is reduced by 3.6 degrees, meaning that the uniformity of temperature field distribution of the cable group is increased after using the optimization method.