The invention provides a testing method and device for stone equivalent extreme shear strength. The method comprises the following steps: placing a stone equivalent extreme shear strength testing device in a test pin on the outer side of a stone foundation pit excavated on a rock mass foundation, and applying a push-up load to the rock mass foundation by a jack in the stone equivalent extreme shear strength testing device until a whole rock of the rock mass foundation is damaged by shearing; recording the maximum jacking force Pmax of the jack, the inner diameter D of an oil cylinder, the diameter a of a part which applies the push-up load to the rock mass foundation, the excavation depth h of a foundation pit and rupture angles alpha and beta of two sides of the rock mass foundation; anddetermining rock equivalent extreme shear strength tau s according to Pmax, D, a, h and the rupture angles alpha and beta. By the testing method, the rock equivalent extreme shear strength of joint fissure rock mass foundation can be conveniently and accurately measured in field, the basis is provided for optimal design of a rock build-in foundation of a mountainous electric transmission line, theengineering cost is reduced favorably, and energy saving and emission reduction are realized.