A horizontal oil-free screw vacuum pump with an overpressure exhaust function is characterized in that each screw rotor is a variable pitch rotor, screw lead of the screw rotors decreases gradually inthe axial direction from a pump block intake port to an exhaust port, and the pump herein has a compression ratio of R, wherein R>3 and R=V/V, and wherein V is intake volume of the screwrotor at the pump block intake port within helical angle 2 pi, and V is exhaust volume of the screw rotor at the pump block exhaust port within helical angle 2 pi; the pump block sidewall is provided with relief holes between the pump block intake port and the pump block exhaust port; each screw rotor corresponds to one group of relief holes; the outer side of a pump block is provided with arelief cavity; an inner cavity of the pump block is communicated with the relief cavity outside the pump block via the relief holes; the relief cavity is externally connected with a relief pipe thatis provided with a relief valve; the relief pipe is communicated with a pump block exhaust passage via the relief valve or is directly communicated with air; when gas pressure at the relief holes is higher than exhaust pressure, the relief valve is opened to discharge over-compressed gas.