The invention patent belongs to the technical field of coal mine roadway ventilation, and particularly relates to a coal rock borehole stress measuring device and a measuring method thereof. A cantilever beam structure is utilized to measure stress changes at a point during coal rock stress changes, so that maximum and minimum principal stresses of a plane are calculated, and the principal stressdirection is determined through a magnetic sensor. According to the technical scheme, the coal rock borehole stress measuring device includes a casing, the top of the casing is provided with a waterproof cable clamping head, the bottom of the casing is provided with a front plug, reinforcing tables are evenly protruding on the circumference of the casing and provided with holes, a balance beam sensor is arranged in the casing and connected with a force measuring part, the force measuring part extends out of the casing through the hole in the reinforcing table, each reinforcing table is provided with an elastomer, the elastomer is connected with the force measuring part, and then the measuring method is adopted for measurement.