In order to solve the problem of precise detection of small modulus gear and break through the limitations of traditional detection methods such as measuring force, measuring diameter, gear thickness, easily damaged probe, high cost and low efficiency, this paper studies the principle and implementation method of precise detection of small modulus gear based on polychromatic laser confocal ranging technology and four-axis linkage motion platform. An adaptive optimal measurement path planning method based on the fixed distance and constant light intensity servo control method is proposed, which reduces the influence of the measurement spot size and incident angle on the measurement accuracy. The three-dimensional topological measurement method of small modulus gear tooth surface in non-contact measurement process and the adaptive matching calculation and analysis model of three-dimensional measured profile and theoretical profile are studied. The unification of single tooth surface matching and multi-tooth surface matching effect is realized, and the influence of clamping error on measurement accuracy is reduced. Finally, the accurate detection of gears whose modulus is less than or equal to 0.1 mm is realized. The measuring platform can measure gear part above ISO1328-2013 class 3.