This paper introduces the calculation method of using the practical torque formula of the electric motor to calculate the resistance value of the liquid starter in cement plant for large equipment like large mills. Taking a ball mill as an example, the comparative analysis was performed for the accuracy of the initial external resistance value and the starting time of the motor using motor practical torque formula and reference calculation formula in the liquid starter manual. The resistance value calculated by the motor‘s practical torque formula is more accurate, and the motor start time is shorter, which can be selected and determined, make full use of the motor‘s electromagnetic torque characteristics, reduce the starting impact process, and make the equipment run more stable. And at the same time, it can effectively improve the heating situation of the liquid starter and shorten the commissioning time. It should be noted when calculating the starting time of the square-torque load such as the fans, the starting time can be approximately calculated by using the average load torque.