The utility model discloses a hydraulic control type downhole releasing device. The device mainly comprises an upper joint, a positioning steel ball, a piston, a pressing sleeve, a spring, a lower joint, an adjusting nut, an adjusting steel ball, a sealing ring and a pressure building steel ball, the device is characterized in that the upper connector and the lower connector are connected througha positioning steel ball, the positioning steel ball is limited by a piston to avoid falling off, the piston limits axial movement of the piston through an adjusting steel ball, an adjusting nut is fixed to the upper connector, the adjusting steel ball acts on a pressing sleeve, and a spring is arranged between the pressing sleeve and the adjusting nut; when releasing operation is executed, a pressure building steel ball is thrown into the tubular column, a drilling pump is started to build pressure, after the piston moves downwards to the bottom of the upper connector, the positioning steel ball slides out of the arc groove, the upper connector is separated from the lower connector, and then downhole tool releasing can be achieved. The well completion device is simple in structure, convenient to operate and suitable for well completion operation.