The invention provides an isolation structure and a forming method thereof. The forming method of the isolation structure comprises the steps: sequentially forming a first liner oxide layer and a first hard mask layer on the first surface of a semiconductor substrate; etching the first hard mask layer and the first liner oxide layer to form an opening; performing first ion implantation through theopening to form a first ion implantation region in the semiconductor substrate; corroding the first ion implantation region to form a porous silicon layer; and oxidizing the porous silicon layer to form a first isolation part. According to the method, the technological process is optimized, a photomask is saved, and the production cost of enterprises is reduced.