Phase-sensitive optical time-domain reflectometry (ϕ-OTDR) based on coherent detection is one of the most widely used schemes to achieve fiber distributed acoustic sensing. In previous studies, a fairly high data acquisition speed is essential for the phase-measuring coherent ϕ-OTDR, thus leading to a severe computational burden. In this Letter, we first analyze the power spectrum of the beat signal and then propose the use of undersampling theory to reduce the need for high sampling frequency. Then we give the principle of selecting the matched sampling frequency and bandpass filter bandwidth so that the beat signal can be sampled without aliasing. The experimental results show that, when the central frequency of the beat signal is 200 MHz, its phase signal can be correctly demodulated even using a sampling rate as low as 71 MSa/s. This method can be extended to all existing coherent ϕ-OTDR systems with no or only a few modifications on them.