The direct current (DC) transformation of alternating current (AC) lines is expected to alleviate the problem of excessive power supply pressure in some urban lines. It is necessary to study the current capacity and operation voltage to improve the power transmission capacity of the cable line after the transformation. The 10 kV three core AC cable in urban distribution network is used as an example to simulate the DC transformation under three laying conditions of air, direct buried and pipe laying. Firstly, the DC current capacity and suitable operation voltage range of the cable under three conditions are obtained according to the simulation results of temperature and electric field. Then, the maximum transmission power before and after the cable transformation is analyzed based on the current capacity and operation voltage of the simulation. Finally, the temperature field simulation of multi circuit line laying is carried out. The study results show that the power transmission capacity of DC cable which transforms from AC cable is improved, and the improvement of direct buried laying is the most obvious under three kinds of laying conditions.