The establishment of offshore wind power transmission network composed by a number of offshore wind farms becomes an inevitable trend in future.Different topologies of the schemes would directly affect the investment cost of network and the reliability of the system's output power.In order to assess this new power system's reliability,the paper analyzes the reliability of offshore wind power transmission network quantitatively based on the selected reliability assessment method according to the characteristics of such new form power network.Different topologies'failure frequency and failure time are calculated based on Graph Theory,which is achieved through analysis of the network's minimal path sets.The reliability indexes of different topologies are calculated at the end and the qualified scheme are selected.