The invention discloses a fabrication method of a metal oxide semiconductor transistor. The fabrication method comprises the following steps of providing a semiconductor substrate defined with an active region, and sequentially forming a gate oxide layer and a poly-silicon gate on the semiconductor substrate surface of the active region; forming gate light-doped masking layers at two sides of the poly-silicon gate; injecting ions to the semiconductor substrate by taking the poly-silicon gate and the gate light-doped masking layer as a mask to form a light-doped drain region; forming side wall layers on the side surfaces of the gate light-doped masking layer; performing deposition to form second poly-silicon layers on the semiconductor substrate surfaces at two sides of the gate by taking the poly-silicon gate, the gate light-doped masking layer and the side wall layers as masking; and carrying out source-drain ion injection through the second poly-silicon layers, and forming a source and a drain in the semiconductor substrate. With the adoption of the scheme disclosed by the method, the parasitic capacitance of a metal oxide semiconductor (MOS) transistor can be reduced.