Abstract: In view of problems of complex design in hardware and big measuring error in software respectively existed in frequency measuring methods with hardware and software, the paper proposed a power frequency measuring method based on linear interpolation. Firstly, the method finds two nearest sampling points to the first zero-crossing point in half-wave and uses linear interpolation to calculate T1 between the zero-crossing point and the later sampling point. Then it finds two nearest sampling points to the next zero-crossing point in half-wave and uses linear interpolation to calculate T3 between the zero-crossing point and the former sampling point. And then it calculates T2 between the first sampling point and the latest one. At last, it calculates power frequency f by adding T1, T2 and T3. The test result showed that measuring error of the method is ±0.05 Hz with simple calculation process.