The invention provides an in-situ shearing and permeability increase determining device. The device comprises a fracturing kettle which is provided with a filling hole and a draining hole in the sidewall as well as seeping-in and seeping-out holes in the top and bottom walls respectively, an upper fixed clamping rod and an upper movable clamping rod are arranged in the top wall of the fracturing kettle, a lower fixed clamping rod and a lower movable clamping rod are arranged in the bottom wall of the fracturing kettle, and a rock sample can be clamped between the upper and lower fixed clampingrods and between the upper and lower movable clamping rods; a return spring is arranged between the bottom wall of the fracturing kettle and the lower movable clamping rod; the lower movable clampingrod has a freedom degree of downward movement; the upper end of the upper movable clamping rod is used to bear an axially downward active force; and the top and bottom walls of the fracturing kettleare provided with seeping-in and seeping-out grooves respectively, and the seeping-in and seeping-out grooves are communicated via the upper movable clamping rod, the fractured surface of the rock sample to be tested and the lower movable clamping rod. Thus, the rock can generate a natural crack by shearing damage in the in-situ condition, and the shearing slip and permeability increase of the natural crack can be researched.