The force controller of series elastic actuator (SEA) is an important issue, because it determines the output performance of SEA (the bandwidth and the resonant peak). However, the existing force controllers of SEA attach little importance to the determinants of the performance. This paper develops four different kinds of force controllers and selects the optimal controller by comparing their capabilities. The comparison demonstrates that the parameters of dynamical models are key factors affecting the output performance. Finally, the experiments reveal that the equivalent mass and the damping coefficient of SEA are determinants. The conclusion can provide theoretical implications to SEA design.