In order to improve the performance of DC speed control system, the DC speed controller was designed as a fractional order PI (FOPI) and was compared with integer order PI for the control characteristics. When determining the FOPI/PI controller parameters, optimization was performed according to different objective functions using the particle swarm optimization algorithm. The influence of the controller parameters determined by different objective functions on the performance of the speed control system was analyzed. Simulation experiments showed that FOPI controller had better control performance. In the design of controller parameters, according to the performance requirements, different objective functions could be selected to optimize the design.