Multiple transmission lines or crossing transmission lines would lead to induced voltage in adjacent lines. Induced voltage of transmission lines on the same tower even reach up to thousands of volts. Thus if a megger is directly used to test insulation and identify phase of a transmission line, there exists high risk for the megger. In this paper, a kind of power frequency filter is designed based on a RC divider. It can effectively reduce power frequency induced voltage to the level that the megger can bear by installing the device between a megger and a tested line. The usage of the device does not have any effect on the results of insulation and phasing tests. Models of megger, power frequency filter and high-voltage double-circuit transmission lines on the same tower were built with PSCAD. The power frequency filter parameters were optimized by system simulation. According to the simulation results, it is a simple and feasible method to apply power frequency filter to the insulation and phasing tests of transmission lines, by which megger would be out of the damage of induced voltage and the test results would not be influenced as well.