In recent years, accidents in protection action caused by the output DC bias signal of a merging unit have been occurring frequently in smart substations. To solve this problem, we need to improve the stability and security of the sampling value output of the merging unit. The reason for DC bias of a merging unit when the sampling circuit is broken is analyzed by introducing the working principle of a merging unit and the design principle of an ADC chip. Based on the sampling value differential principle of a bus protection device, the influence of DC bias output of the merging unit on bus protection is analyzed from the action behavior. By reforming the peripheral circuit of the ADC chip, a solution to reduce the DC bias of the merging unit is proposed. The test system is built to verify the output value of the sampling value and bus protection action. It proves the effectiveness of the DC bias solution for the merging unit. This work is supported by the National Natural Science Foundation of China (No. U1804252).