A quasi-AC Optimal Power Flow (OPF) solution is proposed to solve the OPF problem of a Voltage Source Converter-based Multi-Terminal DC (VSC-MTDC) system. By analyzing the principle of the VSC, an equivalent model considering the loss of the VSC station is established with AC power supply. Constraints are added to the DC grid to equate it to the special AC grid. The energy coupling equation of the VSC is linearized to simplify the difficulty. Based on the above, the power flow of a VSC-MTDC system can be calculated as an AC system. Since the OPF problem of an AC system is non-convex, the OPF model is relaxed to a semi-definite programming model. The test result shows that the proposed method can solve the OPF problem of a VSC-MTDC system. This work is supported by National Key Research and Development Program of China (No. 2018YFB0904503) and Scientific and Technological Project of State Grid Ningxia Electric Power Co., Ltd. (No. JA29DK190001).