As offshore wind farms are getting larger and larger， the research on their collector networks has attracted growing international concerns recently. The switching over-voltages of the collector networks with and without running cable feeders are analyzed using PSCAD/EMTDC software， and the principles of two kinds of switching over-voltages are explained. Then the impact of different design parameters on the switching over-voltage are investigated to provide references for the design of the collector networks and of step-up transformer insulation.