A depressurization device for downhole annulus drilling fluid includes a central shaft and a barrel body sheathed outside the central shaft, the central shaft includes a force transmission shaft and a liquid separation piston; the barrel body includes a spline barrel, an elastic element protection barrel, a connector and a depressurization barrel; the elastic element protection barrel and the connector are sheathed outside the force transmission shaft; an elastic element is arranged between the elastic element protection barrel and the force transmission shaft; the depressurization barrel is sheathed outside the liquid separation piston; a first liquid storage chamber and a second liquid storage chamber are formed; a first inlet passage and a first drain passage are formed on the connector; a second inlet passage and a second drain passage are formed on the depressurization barrel; suction valves are arranged on the first inlet passage and the second inlet passage; and, second drain valves are arranged on the first drain passage and the second drain passage. In the present application, by using the vibration of a drill string as an energy source, the pressure of downhole drilling fluid is reduced.