At present

the researches on the start-up characteristics of modular multilevel converter（MMC） mainly focus on the research and analysis of the start-up method of the converter. The AC/DC voltage and the interactive effect of the two terminal system on charging are not analyzed. Based on the basic structure of modular multilevel converter in Luxi asynchronous interconnection project and the principle of uncontrolled charging on the AC side

the voltage of AC/DC side and the interaction effect between two terminal systems during the uncontrolled charging process are analyzed in detail. The analysis shows that according to the Luxi asynchronous networking system structure

the one terminal uncontrolled charging process will generate the third harmonic on the DC polar line. In the two terminals uncontrolled charging

the distribution of the third harmonic will be determined according to the phase difference between the two terminals of the AC system and the DC voltage will also change according to the phase difference between the two terminal of the AC system; the sum voltage of power modules in the each bridge arm and the DC voltage may even exceed the peak value of the AC line voltage on the valve side.The charging simulation model for VSCHVDC of Luxi asynchronous network project is established in PSCAD/EMTDC. Through the simulation research on the charging model of Luxi asynchronous network project

the research results are compared with the uncontrolled charging process of Luxi asynchronous network project.verified the correctness of the analysis is derived in this paper.