In view of the comprehensive compensation for negative sequence and reactive power in the traction substation， the vector form definition of negative sequence components remaining degree is proposed. On this basis， the RUPQC's mathematical model is established and the comprehensive compensation solution is obtained. The phase angle difference gives the solutions of RUPQC an additional degree of freedom which has the role of optimization for the compensation capacity and the optimum conditions are given. A reactive power and negative sequence components symmetrical decoupling method applicable to the balance traction transformer is proposed. The negative sequence optimal compensation solutions with the priority of reactive power compensation are obtained at the specific limitation of RUPQC's power capacity. Numerical calculation shows that the optimal compensation effects in terms of power quality standards at full load scope are achieved with the proposed control strategy. Consequently RUPQC's cost-effective and practical value is improved. The simulation testifies the correctness of the control strategy and effectiveness.