A semiconductor structure and a method for forming a semiconductor structure are disclosed. A form of a method for forming a semiconductor structure includes: providing a base; patterning the base, to form a substrate and fins protruding out of the substrate, where each fin includes a bottom fin and a top fin located on the bottom fin, and in a direction perpendicular to an extension direction of each fin, a width of the top fin is less than a width of the bottom fin; and forming an isolation structure on the substrate exposed by a fin, where the isolation structure covers at least a sidewall of the bottom fin, and a top of the isolation structure is lower than a top of the fin. In the present disclosure, a bottom fin with a larger width is formed, to increase the volume of the bottom fin, and the area of a contact surface of the fin and the substrate, and to correspondingly enhance an effect of dissipating heat generated during working of a device to the substrate, thereby improving the heat dissipation performance of the device, and to correspondingly improving a self-heating effect of the device, so that the device performance is further improved.