HVDC transmission system has considerable impact on the surrounding power transformers when the system is running in the unipolar ground mode, which will cause the DC magnetic biasing phenomenon on transformers. This problem would be more serious, after commission and operation of UHVDC transmission system in China. According to the Guangdong power grid under the influence of DC magnetic bias seriously, but little research about the using of blocking device, this paper proposed an optimization scheme about the usage of blocking device combination. Firstly, the subject studied the method of suppressing transformer neutral point DC depending on analysis the mechanism of magnetic biasing, and then found out the changes of power grid after using the capacitance blocking device which is popular used by Guangdong power grid. The particle swarm optimization (PSO) has been used to find a better way to suppress the DC in power grid, and combined with NSGA to solve the mixed integer programming problem. The final data validation of this method is valuable in engineering application.