We have proposed a time-domain extremum phase shift laser ranging method based on ergodic under-sampling to address the issues of high sampling rate and complex hardware implementation in conventional phase shift laser ranging systems. This method is combined with the spline interpolation algorithm to further enhance the range resolution and precision. Compared with traditional frequency domain phase shift laser ranging methods, our method has the advantages of a simpler calculation process, higher measurement precision, a lower sampling rate, and easier hardware implementation. We have set up a phase shift laser ranging system with an FPGA to test the viability of the proposed method. The proposed method has shown superior ranging performance than the conventional DFT frequency domain method. In addition, the use of the spline interpolation algorithm can further enhance the resolution and precision. We hope that this ranging method can provide a reference for reducing the hardware complexity of phase shift LiDAR and enhancing ranging precision.