In conventional line protection the return signal of contact in tripping position relay TWJ is usually employed as a criterion of post-acceleration protection startup after analyzing a mal-operation case of pilot post-acceleration protection， it is clear that the design of early TWJ circuit is unreasonable. It shows that the returning of TWJ contact is usually earlier than the tripping of circuit breaker， which frequently leads to the false input of acceleration protection. Meanwhile， there is defect in protection device software and the distance acceleration component may frequently fulfill the operation condition during open-phase operation mode. Factors mentioned above may cause acceleration protection mal-operation. Improvements on field wiring and protection software resulted from the mutual coordination between the secondary circuit and protection device are put forward.