The DC transformer is an important equipment to interconnect DC grids with different voltage levels, however, the impedance mismatch between source and load subsystem adversely affects the system stability. For the cascade system based on multi-module series-parallel combined DC transformer, the inverse droop control strategies are proposed for the corresponding subsystems to achieve equal power sharing among the individual modules. In order to analyze the stability issue of cascade system, the impedance and admittance models of the corresponding subsystem are derived by establishing the small signal model, and the derived results are verified by MATLAB. Then, the influences of the variation of system parameters on the stability of the cascade system are analyzed by using the impedance stability criterion. Finally, the application effect of the proposed inverse droop control strategies are verified by simulation.