To study the influence of interference power grid performance of wind power and solar photovoltaic on third defense line of power grid, in Ningxia power grid, based on specific configuration in third defense line of power grid, low/ over voltage protection and static var generator (SVG) of wind and photovoltaic power stations. Simulate and analyze effect of interference power grid performance on third defense line of power grid. The interference power grid performances include low/ over voltage protection, with or without SVG. The results show that when the wind and photovoltaic power stations do not have SVG and serious fault occurs in power grid, under-voltage load shedding and splitting devices act successively lead to over voltage, trigger overvoltage protection act then lead to large-scale wind and photovoltaic off grid, large power vacancy causes the frequency drop, trigger under-frequency load shedding device act and a series of chain reaction. When with SVG, SVG can provide voltage support, off grid capacity is less, and it will not cause the subsequent frequency problem. Finally, put forward reasonable suggestions to enhance interference power grid performance of wind and photovoltaic.