A method for forming a semiconductor structure comprises the following steps: providing a semiconductor substrate, wherein at least one isolation structure is formed in the substrate; forming first grooves of which the depth is smaller than that of the isolation structures in the substrate on the two sides of the isolation structures; forming a first semiconductor material layer flush with the semiconductor substrate in the first grooves; forming second grooves in the first semiconductor material layer, wherein the bottoms of the second grooves expose the first semiconductor material layer; forming a transition layer higher than the substrate on the first semiconductor material layer in the second grooves; adopting first chemical mechanical polishing to remove the part of the transition layer higher than the substrate, wherein first chemical mechanical polishing includes first coarse polishing and first fine polishing, the upper portion of the part of the transition layer higher than the substrate is removed through first coarse polishing, and the lower portion of the part of the transition layer higher than the substrate is removed through first fine polishing; forming a second semiconductor material layer on the remaining transition layer; and performing N-type doping on the second semiconductor material layer. By adopting the method of the invention, the performance of the active region of the semiconductor device is improved.