The invention provides a fin type field effect transistor base body preparation method, and includes: providing a semiconductor base body, and forming a patterned mask layer on the semiconductor base body; using the patterned mask layer to etch the semiconductor base body to form a fin-shape structure; covering the semiconductor base body and the fin-shaped structure with an oxide layer and a nitride layer in sequence; grinding the oxide layer, the nitride layer and the patterned mask layer through chemical mechanical grinding till the fin-shaped structure is exposed; partially removing the oxide layer on two sides of the fin-shaped structure; epitaxially growing the exposed fin-shaped structure, so as to form an improved fin-shaped structure with a larger upper end and a smaller lower end; removing the nitride layer; and further partially removing the oxide layer so as to reduce the thickness of the oxide layer, thereby forming a fin-shaped semiconductor base body structure whose cross section is omega-shaped.