The invention provides a preparation method of an active region. The problem of a graphical photoresist layer and polymer residues generated by shallow trench etching can be completely solved by implementing the technologies of first performing ashing treatment and then performing wet method photoresist removal for multiple times, so that the key dimensions of the active region are ensured, the effects of subsequent round corner process and filling process of a shallow trench isolation structure are improved and the yield of devices is finally improved.