In order to improve the wellbore stability of hard-brittle shale，the plugging ability of drilling fluid was usually emphasized，but the effect of drilling fluid hydration on mechanical strength of shale was not considered. Based on traditional drilling fluid plugging performance evaluation method，the rock mechanical strength retention ability of drilling fluid was investigated. The effect of hydration of drilling fluid on rock compressive strength，tensile strength and pressure increment of formation collapse was carried out. And a set of evaluation method for wellbore stability of hard-brittle shale was developed. The results showed that the excellent wellbore stability should have a low penetration plugging filter loss，excellent ability to delay the delivery of pressure，high plugging rate for core cracks，and strong rock mechanical strength retention ability，i. e. ，the decrease of rock mechanical strength was minimum，and the increment of collapse pressure was the lowest after drilling fluid immersion. It was suggested that PPT permeability plugging test，pressure transfer test and core fracture plugging test should be carried out initially， the drilling fluid formula with better performance should be selected，and then the influence experiment of drilling fluid on rock mechanical strength should be carried out，combined with the impact analysis of drilling fluid on collapse pressure and fracture pressure. Finally，the optimum formula of drilling fluid with good wellbore stability could be further determined. From the perspective of rock mechanics，this set of evaluation methods comprehensively estimated the wellbore stability performance，and provided reference for the optimization of drilling fluid.