A rotary steering drilling tool drilling fluid pulse signal downloading device comprises a drill pipe and a stand pipe connected with the drill pipe through a water hose. The upper end of the stand pipe is provided with a pressure sensor, the portion, under the pressure sensor, of the stand pipe forms a bypass through a tee-joint, and the bypass is provided with a downloading device body; the downloading device body comprises a pneumatic control valve which is connected with a pulse controller, and the pulse controller is connected with a human-computer interface of a ground computer system; an outlet pipeline of the pneumatic control valve is inserted into a mud tank; the stand pipe is connected with a mud pump through the tee-joint, and an inlet pipeline of the mud pump is inserted into the mud tank; the pressure sensor and the pulse controller are connected with the ground computer system through an RS-485 or wireless transmitting and receiving device. The rotary steering drilling tool drilling fluid pulse signal downloading device achieves transmission of a ground downloading instruction to a downhole tool and is simple, reliable and accurate.