In recent years,offshore wind power has developed rapidly. It is the most effective way to transmit the wind power far away from the coast through the voltage source converter based high voltage direct current (VSC-HVDC) transmission system. The offshore wind power is usually transmitted by cable,which makes the fault characteristics of VSC-HVDC transmission system more complex than that of overhead transmission system. Based on a symmetrical monopole topology of two-terminal VSC-HVDC offshore wind power transmission project,the mathematical model of the transmission system which contains cables and VSC-HVDC system is simplified. The fault characteristics under typical faults such as single-phase grounding of AC system at converter valve side and single pole grounding of DC cable line are analyzed. Based on the superposition principle,the fault mechanism is analyzed in detail. According to the fault sources of different fault types,different fault circuits are equivalent. The complex influence of the significant capacitance effect of long-distance cables on the fault characteristics of VSC-HVDC systems is obtained. A real time digital simulation model of offshore wind power transmission system through VSC-HVDC is built. The fault mechanism is verified through simulation analysis.