The invention discloses an external labyrinth type oil-gas pre-separator which is arranged on a cylinder block cover plate. The external labyrinth type oil-gas pre-separator comprises a prefilter baseplate, a prefilter shell, a plurality of transverse baffle plates and a longitudinal baffle plate. The prefilter base plate is arranged clinging to the cylinder block cover plate, the prefilter shellis buckled to the prefilter base plate, and the prefilter base plate and the prefilter shell form an accommodating space which has a certain thickness and is relatively closed; the transverse baffleplates are successively and parallelly arranged on the front side walls of the prefilter base plate and the prefilter shell in a staggered manner, and the transverse baffle plates form an S-shaped channel in the accommodating space; the longitudinal baffle plate is arranged at the rightmost end of the front side wall of the prefilter shell and is vertical to the transverse baffle plates, the longitudinal baffle plate is connected to the rightmost transverse baffle plate among the transverse baffle plates and forms a cavity with the upper side wall and the right side wall of the prefilter shell, and the cavity communicates to the S-shaped channel. The external labyrinth type oil-gas pre-separator can separate oil from gas quickly and smoothly through the internal S-shaped channel.