With the characteristics of low loss, low harmonic content and large transmission capacity, VSC-HVDC has become the inevitable development direction of large capacity and long-distance transmission of offshore wind farms. Due to wind farms and VSC-HVDC systems are contains a lot of power electronic devices, the fault characteristics of power collecting lines in VSC-HVDC connected large-scale offshore wind farms will change significantly. Considering the wind farm side and VSC-HVDC side have a great different control strategies, the fault characteristics of both sides are studied, and the vector expression of the fault current on both sides of high voltage collecting line is derived. The electromagnetic transient model of offshore permanent magnet synchronous wind farm is built in EMTP-RV, and the correctness of the proposed fault current expression is verified by simulation.