The invention discloses an oil-gas separation device for an engine. The oil-gas device comprises a gear arranged in the engine, wherein a gear shaft on the gear is closely matched with a supporting bush; the supporting bush is installed in an engine gear chamber; the gear is meshed with other gears of the engine; the gear is provided with a first through hole in radial direction; the gear shaft is axially provided with a third through hole; the third through hole is communicated with the third through hole; and the third through hole is communicated with the external part of the engine via a path. In the oil-gas separation device for an engine, the linear speed of rotation of leak gas of a piston in the first through hole is higher because the gear rotates at a high speed when the gear operates, thereby achieving high oil-gas separation effect and high separation efficiency, The oil-gas separation device for an engine has the advantages of simple structure and lower cost.