Phase laser ranging is an important means of absolute ranging and an important guarantee for large-scale precision measurement. One of the most effective ways to improve the precision of phase laser ranging is to increase the laser modulation frequency and use high-performance devices to achieve high-frequency sampling analysis. However， the maximum sampling frequency of high-performance devices is limited. To solve the problem that existing devices have difficulty in the sampling of high modulation frequencies， the feasibility of an undersampling method for phase ranging was analyzed and verified. The advantages of all-phase fast Fourier transform （apFFT） analysis was examined to improve the precision of phase laser detection. Based on this idea， the method of undersampling and apFFT was developed， and a phase detection system for laser phase ranging was constructed. When the modulation frequency is 201 MHz and the undersampling frequency is 100 MHz， the system phase discrimination accuracy is higher than ±0.04°， and the corresponding ranging accuracy is approximately ±0.08 mm. The experimental results show that the phase ranging method based on undersampling and apFFT has the comprehensive advantages of high accuracy and strong anti-interference ability， making it valuable for scientific research and engineering applications.