Abstract: In order to explore the effect of the power difference between arms of dual buck modular multilevel DC transformer（DBMMDCT） when the coupled submodule inductor parameters are unbalanced, this paper builds up the common-mode and differential-mode circuits of DBMMDCT and divides the arm transformation power into common mode and differential mode parts to figure out the quantitative relationship between arm power difference and submodule inductance difference. Simulations are made to verify the theoretical analysis, and the result proves that the asymmetric submodule inductance can cause voltage bias in the arm inductance voltage, which can increase circulating current ripple and reduce operational stability.