Abstract: Based on the characteristics of low light energy loss, relatively simple assembly, relatively easy debugging, and high system resolution, the critical angle focusing technology can be introduced to amplify the extracted focusing signal, improve the detection sensitivity and the smoothness of the focusing curve, and at the same time achieve a larger focal plane detection range, reducing the focusing error affected by the unevenness of the substrate itself. This paper first starts from the basic principle of the differential critical angle focusing technology and obtains the relationship between the defocus amount and the defocus signal through the Fresnel formula and the Newton formula. Secondly, a differential critical angle focusing verification system is built, a four-quadrant photodetector collects the defocus signal of a single critical angle prism, and the defocus signals received by two vertically placed four-quadrant photodetectors are differentially calculated to obtain the relationship between the differential K value and the defocus amount z. The experimental results show that when a wavelength of 532 nm laser and a numerical aperture of 0.3 projection objective are used, the focusing linear range can reach 22 μm. When a numerical aperture of 0.45 projection objective is used, the focusing linear range can reach 14 μm, and the resolution of the differential critical angle focusing method can reach 25 nm.