This invention is to provide a fuel-air separation structure improvement for engine blow-by, which features an extra oil recirculation tube set up in between the bottom of the fuel-air separator and the engine, where the flow connection of the oil recirculation tube is controlled by a check valve, and the direction of the check valve is enabled only in a unidirectional way, pointing from the fuel-air separator to the crankcase of the engine, whereas the reverse direction is being cut off. By means of the aforementioned structure, the problems of carbon deposits in engine and exhaust pollution due to an excess of oil along with blow-by in the fuel-air separator can thus be substantially improved.