The invention discloses a manufacturing method of a shallow trench isolation structure, which comprises the following steps: successively forming a pad oxide layer, a silicon nitride layer and a photoresist layer on a semiconductor substrate; after performing an exposure development technology, taking the photoresist layer as a mask to etch the pad oxide layer, the silicon nitride layer and the semiconductor substrate to form a shallow trench; etching back the silicon nitride layer; using an on-site steam generation technology to form a lining oxide layer on the inner wall of the shallow trench and an active area exposed outside due to etchback; filling an insulation layer into the shallow trench; and removing the silicon nitride layer and the pad oxide layer to finally form the shallow trench isolation structure. The above method is favorable for forming symmetrical round vertex angles on the STI structure and increases the thickness of the oxide layer on the round vertex angles so as to lower the adverse effect of a pit on the electrical behaviour of semiconductor components.