Kunliulong UHVDC is the world's first hybrid multi-terminal UHVDC project in which the VSC station adopts the series structure of high and low converter. Before starting，it needs to charge the high and low converter in turn. As for the tripping event of Liuzhou converter station during charging process of high and low values /value group，this article first points out that the later charging converter jassively charged by DC side is a special phenomenon of VSC-UHVDC. Combined the mechanism analysis of this special phenomenon and the charging logic of valve control system，the reason of tripping event is found. Finally，based on the above analysis，the corresponding improvement measure is put forward and verified through RTDS simulation. The proposed scheme has been successfully applied to Kunliulong HVDC project. It has provided a guarantee for projects debugging and subsequent safe and stable operation，which also has an important reference value for future VSC-UHVDC projects.