As the phase discrimination error of a phase laser ranging system effects its ranging accuracy, this paper optimizes the designs of a preceding stage (shaping circuit) and a subsequent stage (low-pass filter) in the phase detector. For the shaping circuit, a comparator with hysteresis was designed to avoid the multiple-triggered phenomenon caused by the noise on GND for open loop filters, and stabilizes the output of phase detector. For the low-pass filter, two typical amplifier/filter circuits based on MC4044 and their defects were introduced, a 4th-order low-pass filter was designed by using FilterPro, and its advantages were presented. Experimental results indicate that the multiple-triggered phenomenon is eliminated by using the hysteresis comparator designed, and the rise-time of output square wave falls to 108 ns from 1.66 μs as compared with that of a traditional opened-loop comparator. Furthermore, the 4th-order filter designed in this paper improves the linearity and sensitivity of the DC output, the linearity(R2) increases from 0.908 3 to 0.999 9, and the convention gain improves by 96.5% as compared with that of the typical designs of amplifier/filter circuits. Moreover, the peak-to-peak value of the interference signal on a Low Pass Filter(LPF) DC output decreases from 50-230 mV to 10-20 mV, which reduces the phase detection error effectively.