Overtemperature due to accumulation of heat in a crowded cable trench accelerates the aging of cable insulation materials, and leads to shorter cable life. In order to assist maintenance engineers with additional information when increase in circuit ampacity demand requires installation of additional cables, an optimization method is proposed to reallocate the current of selected cable circuits using spare cable pipes in a cable trench. The objective is set in this paper as to limit the operating temperature and extend average service life of cables. Based on the Finite Element method, the temperature field in an underground cable trench which has 24 cable pipes, of which 20 have been occupied with operational cables, is simulated. According to previously reported cable aging theory, the stochastic electro-Thermal degradation accumulation in cables is modeled, and the average service life of cables in the trench is assessed. An optimization method of reallocating the current by adding cables in spare pipes is established. It is found from results of cable average service life assessment that, by adopting the proposed optimization method, the average service life of the cables in the trench can be increased by 2.2 years while the maximum conductor core temperature among cables in the trench is reduced by 1.9 °C, comparing with the redistribution which was adopted without applying the proposed method.