The invention discloses a vertical oil-free screw vacuum pump with an overpressure exhaust function. A screw rotor is a varying-pitch rotor with a double-end holohedral line, and the screw lead of thescrew rotor is gradually reduced along the axis direction from an air suction port to an exhaust port of a pump body; the compression ratio of the screw vacuum pump is set as R which is greater than3 and is equal to Vin/Vout, wherein Vin is the air suction volume of the screw rotor at the air suction port of the pump body within 2(Pi) helical angle, and Vout is the exhaust volume of the screw rotor at the exhaust port of the pump body within the 2(Pi) helical angle; a pressure release hole is formed in the side wall between the air suction port and the exhaust port of the pump body; each screw rotor is corresponding to a group of pressure release holes, and each group contains two rows of pressure release holes; a pressure release cavity is formed outside the pump body; an inner cavity of the pump body is communicated with the pressure release cavity outside the pump body through the pressure release holes; the pressure release cavity is externally connected with a pressure release pipe; a pressure release valve is arranged on the pressure release pipe; the pressure release pipe is communicated with a pump body exhaust channel through the pressure release valve or is directly communicated with the atmosphere; when the gas pressure at the pressure release hole is higher than the exhaust pressure, the pressure release valve is opened to exhaust the over-compressed gas.