Abstract: In order to accurately locate the focused spot of the laser direct writing system on the surface of the element to be machined in real time, a synchronous phase-shifting microscopic interference focal detection method based on wave aberration criteria was proposed, which realized the real-time detection and adjustment of the defocusing amount. The focal detection optical path was introduced in the direct writing system, and shared the same objective lens with the direct writing optical path. The Linnik synchronous phase-shifting microscopic interference focal detection system was constructed to extract the wave surface phase information containing the defocusing amount. Then the magnitude and direction of defocusing amount was analyzed from the wave aberration data of objective lens with large numerical aperture (NA). The simulation results verify the correctness of defocusing calculation method based on wave aberration criteria, when NA≥0.5, the defocusing detection sensitivity can reach to 4 nm. The experimental results verify the feasibility of synchronous phase-shifting microscopic interference focal detection method, and the precision can reach within 10 nm, which can meet the high-precision measurement requirements of laser direct writing.