A power transmission system of a vehicle and a gear-shift control method are provided. The power transmission system includes a first drive assembly (101) and a second drive assembly (102). The first drive assembly includes a first automatic transmission (2) and a first motor (1) connected with the first automatic transmission (2) for outputting power to two wheels of the vehicle. The second drive assembly (102) includes a second automatic transmission (2000) and a second motor (1000) connected with the second automatic transmission (2000) for outputting power to the other two wheels of the vehicle. The first and second automatic transmission are configured to be shifted to a preset gear or a gear adjacent to the preset gear, such that when one of the first and second automatic transmission is shifted to the preset gear, the other one of the first and second automatic transmission is shifted to the preset gear or the gear adjacent to the preset gear.