substation bilateral tripping protection is one of the important protection of DC traction, which plays an important role in the security of power supply equipment and personal safety. But in the DC system protection operation, there will be joint protection misoperation, resulting in the neighboring substation tripping and interruption of power supply in large range. In order to fundamentally solve the problem of debugging the misoperation of protection, taking Nanjing Metro as an example, this paper analyzes the reason and mechanism of joint protection misoperation, proposes the "method of voltage compensation" to optimize relay element design of protection circuit and decrease the contact resistance value of protection circuit. The running after construction and transformation proves that the method is simple and reliable and has low cost, which effectively solves the tripping of misoperation of protection loop.