This paper presents a method based improved particle swarm optimization optimizing PI control parameters. When the load changes, parallel active power filter, in the traditional PI control strategies, can not well control DC voltage fluctuations leading the deterioration of its dynamic performance. This paper proposes the improved particle swarm algorithm by increasing symbolic function of inertia weight parameter in particle swarm algorithm, and changing position of the random number in the speed update equation. The improved particle swarm algorithm can avoid "premature" or fall into local optimal solution, so it could get a more accurate solution set. Using improved particle swarm algorithm to solve PI controller optimization objective function, to get a more accurate PI control parameters. Simulation and experimental results show that mutations in the case load, the overshooting of an capacitor DC voltage compare with the traditional PI control reduces 5.5%, compared with Particle Swarm Optimization reduces 1.5%. Based on the simulation analyses and experimental results, it has been proved that this method can realize a comprehensive optimization.