The utility model provides a hydraulic pressure of discharge capacity control in pit trigger mechanism, it includes: the casing, it has the axial passage who link up, trigger the piston, its inside cavity and can axial displacement ground holding in axial passage, trigger the concave formation annular groove of establishing in upper portion of piston, the center pin, its inside cavity and holding are in axial passage, and the lower extreme of center pin can be inserted and locate in the annular groove, axial motion spare, it can overlap the upper end outside of locating the center pin and lie in the casing. The utility model discloses play the effect of hydraulic selector in underground operation, when it is opened or closes, do not arouse extra pressure loss, can avoid the influence of drilling fluid flow oscillation effectively, improved the stability of moving with the continuous downhole tool of axial motion spare.