A signal’s amplitude and frequency can be measured by sampling and analysis. However, according to Nyquist sampling theorem, it is difficult to capture highfrequency signals with the method of Oversampling limited by the sampling rate. To obtain a signal’s amplitude and frequency by a low sampling rate, design a method within which the amplitude and frequency of a signaltone periodic signal can be measured by Undersampling. Take a selfdeveloped data acquisition card as the hardware platform, take advantage of a sampling rate of 100 MSa/s to measure the amplitude and frequency of a signaltone periodic signal which is located in a bandwidth of 100 MHz to 150 MHz by FFT operation and curvefitting. The test result makes clear that this method has high accuracy. In addition, with this method, there is no need to recover the original signal, and the hardware is easy to implement, the software bears small amount of calculation as well. This method is particularly suitable for measuring the amplitude and frequency of a signaltone periodic signal which is located in a narrowband.