The invention belongs to the technical field of laser additive manufacturing, and particularly relates to an automatic laser machining forming accuracy regulating method based on a phase transition size effect. The laser 3D printing technology is adopted, Fe-based composite powder with austenite-martensite phase transition transforming is used as 3D printing dedicated powder, when to-be-machined workpiece geometric model hierarchically-discrete designed two-dimension section (x, y, z) information pre-stored in a laser 3D printing technology database is printed layer by layer,each layer of actual two-dimension section(x

, y

, z

) obtained after printing is subjected to dimensional accuracy controlling until forming accuracy of the layer meets requirements,and then next-layer machining can be continued. According to the automatic laser machining forming accuracy regulating method based on the phase transition effect, super-high accuracy forming of complex thin-wall iron-based parts can be achieved, meanwhile the complex thin-wall iron-based product performance can be improved, and the product qualification rate is increased.