Abstract: A calculation method for the allocation and sizing of active power filters (Active Power Filter,APF) in un- derground power system is proposed in this paper,considering the severe harmonic pollutions of the underground coal mine power system. The method suppresses the harmonic pollutions of the underground coal mine power system and al- so minimizes the investment cost largely. A discrete APF time domain model based on instantaneous power theory and an optimization model based on both fixed cost and unit cost are established. A particle swarm optimization is employed to solve the optimization problem. The method presented ensures that the optimal solution is practical. Based on the simulations results,the correctness of the method for the allocation and sizing of active power filters is verified.