The traditional method of frequency weighting is realized by building up the hardware filter network, and the test accuracy is easily affected by the uncertainty of electronic components. In order to overcome the disadvantages of traditional method, such as complicated hardware and instable test, this paper proposes a method to achieve frequency weighting on the virtual instrument platform, describing the principles and methods of achieving the frequency weighting by the FFT analysis in detail and providing the corresponding block diagram. The measurement results of A and C weighting are verified by the way of inputting analog signals. The experimental results show that the measurement accuracy of the A and C weighting have reached the testing standards of the first class level of sound level meters. The proposed method is simple and flexible, and it can simplify the hardware circuit and satisfy the requirement of the test precision.