Conduit cables are widely used in modern urban power distribution network. Ampacity and temperature rises of cable have close relationship with reliability. In order to explore the relationship between cable ampacity and the cable lying condition, combined with the loss of cable, the heat generation and cooling process of cable channel is analyzed. With the use of COMSOL Multiphysics software, this paper simulates ampacity and temperature distribution of the 8.7/15kV YJV 1×400 three-loop XLPE power cable laying in normal case and irregular case respectively. FEM method is used to do the calculation during the whole simulation period. Cable ampacity relies on the concentration degree of cable bundle， the higher the concentration degree is, the smaller the ampacity is. The position of the cable bundles also decides the ampacity of cable and temperature distribution of cable channel. Therefore, it is necessary to follow the procedures requirements to lay cable bundles as multi-layers in the trenches, which can lower down the operation temperature and extend the life expectancy of cables. Laying cable bundles randomly in the derground of trenches is an improper way.