The quality of laser spot is studied in order to improve the precision of distance and thickness measurement system based on laser diode-position sensitive device (LD-PSD) laser triangulation. Static distance measuring experiments with different powers of laser diodes are conducted. The experimental results show that drift of the center of spot intensity increases measurement errors of the system in long-term continual work. The drift is mainly caused by high-frequency noise and spot displacement. For these two factors, an optical system based on the pinhole filter and beam splitter with prisms to improve the quality of laser spot is proposed, which is optimized and simulated by CODEV. Static distance measuring experiments are conducted again after adding the designed optical system to the rangefinder. The new experimental results prove that the precision of the system is improved from 25 μm to 8 μm. The design improves the practicability of laser triangulation measurement system for distance and thickness measuring in long-term continual work based on PSD.