For small dimension non contact measurement,especially small internal curved surface measurement,an optical focus detection system based on a confocal microscope combination with a gradient index len is presented.This paper describes the measurement principle and structure,and optimizes the key technique in the system.By the design of a gradient index lens,pinhole,light source and detector,high precision and small optical focus detection can be realized.The feasibility of the differential confocal system is experimentally verified and the axial resolution is up to 10 nm.