The invention discloses a method for guaranteeing bearing safety of a stope supporting system in condition of a soft and weak bottom plate, comprising the following steps: (A) determining the type of the bottom plate according to a uniaxial compressive strength of the bottom plate; (B) performing a field measurement by means of a method for actually measuring specific pressures of the bottom plate at different places in an entire lane on the spot; (C) coupling a relation curve between a specific pressure value P(x) of the bottom plate and a thickness value of bottom coal; (D) determining the maximum specific pressure strength of the bottom plate in a critical state; (E) calculating a normal pressure of a fore support of a working face; (F) obtaining the specific pressure value of the bottom plate, when the entire lane is at an arbitrary position, namely arbitrary thickness of the bottom coal, based on measurements and calculations, and making relative preparations in advance when the fore support is advanced to the position. The invention also discloses a full-automatic specific pressure measurement vehicle capable of realizing circulating fluid injection for actual measurement in the field. Results obtained by the method disclosed by the invention can serve for the production working face, and meanwhile bottom coal recovery rate can be increased.