The different levels of DC overvotage are happened when online-switching of three power transmission modes is operated in Yongfu HVDC project. In order to research the influence of online-switching operation on DC overvoltage, based on the simulation of Real-Time Digital Simulator (RTDS), the online-switching tests are done at different DC power levels. According to the test data, the law and reason of DC switching overvoltage are studied, the safe power level value can be determined, and the following conclusion is drawn:it’s proportional relationship between the level of DC switching overvoltage and the DC transmission power, higher DC switching overvoltage can be avoided when the online-switching operation is under the safe power level. The research results provide reference values for the practical operation of the project.