As the most basic optical element in the optical system,the center thickness machining precision and assembly precision of the lens directly affect the imaging quality of the optical system.Kinds of non-contact measuring methods that have been proposed in recent years are introduced.The working principles and developing trends of the imaging method,axial dispersion measuring method,differential confocal measuring method,low coherence interferometry measuring method are described,as well as their own advantages and disadvantages.These methods can satisfy the precision requirement of the optical system,and the low coherence interferometry method provides the highest precision.The axial dispersion method and the confocal method comes second,and the imaging method is the last one.The developing direction of the lens gap measurement technology is discussed.