The invention discloses a shallow trench isolation structure and a forming method thereof. The shallow trench isolation structure includes an upper insulating part and a lower insulating part, the lower insulating part includes a first insulator and an insulating layer located around the first insulator, the upper insulating par includes a second insulator and a buffering layer located around the second insulator, part of the buffering layer is located between the first insulator and the second insulator, and a peripheral wall of the buffering layer is flush with a circumferential wall of the first insulator.