Abstract: In order to analyze the length effect of working face of weakly cemented soft roof stope in western China, based on the analysis of rock characteristics by mechanical test, XRD test and electron microscope scanning, the length effect of working face is systematically studied by combining theoretical analysis and numerical simulation. The study show that the main sandstone layer in 2-2 coal seam roof of Selian No.2 mine has low strength, weakly cemented and poor deformation resistance, which is easy to fall down and compact. The caving height of overburden is positively correlated with the change of working face length. The larger the face length, the higher the caving height, the shorter the plate structure formed in the process of advance, and the smaller the corresponding compression step. The caving height and compression step under different working face lengths are simulated by UDEC. When the length of working face changes from 160 m to 300 m, the caving height rises from 18.18 m to 29.05 m, the initial compression step decreases from 27.8 m to 25.9 m, the periodic compression step decreases from 14.4 m to 10.5 m, and the caving height and step gradually becomes stable after the length of working face exceeded 240 m. Then the pressure arch model is established based on the properties of weakly cemented soft rock, and the solving formula of support load under different working face lengths is obtained by combining with the analysis of length effect of working face. It is determined that the support force also increases with the increase of face length and becomes stable to a certain length. The field measurements are basically consistent with the theoretical analysis, which provides a useful reference for the selection of working face length and support under similar geological conditions.