Ampacity is an important parameter in power cable operation. In order to provide a reference for the operation of ultra-high voltage cables laid in tunnel,a three-dimensional geometric model is established to simulate the thermal-fluid coupling by COMSOL Multiphysics according to the actual cable tunnel structure and internal cable arrangement. The distribution of temperature and wind velocity under different operating modes and environmental conditions is analyzed based on finite element method. The ampacity of tunnel ultra-high voltage cables is calculated. It is observed that the highest temperature appears at the conductor. The temperature decreases gradually along the radial direction of cable. Temperature and wind velocity at the exit are increased compared to the entrance. With the increase of current,the influence of cable heating on ambient temperature also increases. The steady-state load capacity of cable for double-loop or four-loop laying is higher than that for eight-loop laying. Cable surface temperature decreases with increasing ventilation rate.