This paper presents a self-reference focusing digital holographic microscopy method based on common-path shearing. The method employs a dual-polarization grating shearing device that utilizes the polarization grating’s spectral properties to replicate the object wave carrying sample information. By adjusting the distance between the dual-polarization gratings， the spacing between the micrograph and the holographic interference image on the camera can be flexibly controlled， enabling simultaneous recording of both. Through recording multiple images from defocus to focus to defocus， the Brenner gradient method is applied to determine the optimal focus， achieving self-reference focusing. The ASM is then used to reconstruct the interference region’s phase at the best focus. Experimental verification， using a phase resolution plate， demonstrated a measured phase height of 0.336 μm with an average absolute error of 0.023 μm. The proposed method features high accuracy， stability， and a compact design.