This paper provided the double-loop regulator design and parameter calculation for Buck chopper on a certain HXDx locomotive by using typical I type and II type system design, which improved the bandwidth of inner current loop and stability of outer voltage loop through reallocation of zero and pole. Compared to single-loop control, the double-loop control could configure the system parameter more flexible, which could improve the stability and dynamic performance. The simulation and experiment showed obviously that double-loop control was superior to single-loop control especially in loading and unloading, which validated the feasibility of the calculation.