Formation method of fin structure and formation method of FinFET device

The invention provides a fin structure forming method and a FinFET device forming method, and the fin structure forming method comprises the steps: providing a substrate which is provided with at least two fin groups which are arranged at intervals; forming an isolation film, and filling the fins and the fin group; forming a patterned mask layer, wherein an opening of the patterned mask layer exposes the isolating film between the fin groups; breaking molecular bonds of at least part of the isolating membrane by using the patterned mask layer; performing heat treatment on the isolating membrane; and removing the isolating film at a preset depth to enable the exposed fins to be fin structures. According to the heat treatment method, the molecular bonds of at least part of the isolating membranes among the fin groups are broken, so that the expansion rate of the isolating membranes among the fin groups during heat treatment is adjusted, the stress of the isolating membranes on the two sides of the fins in the f