The invention discloses a preparation method of a section mutation eccentric cylinder forging. The preparation method comprises the following steps: the structure of a blank is determined according to a final forging of a section mutation eccentric cylinder, and need to satisfy the following two conditions: (1) the blank has a section mutation eccentric part, and the section mutation eccentric part of the blank need to totally cover a spherical cylinder forming cavity of a final forging mold, so that the spherical cylinder forming cavity satisfies reverse filling formation during final forging, and the folding generated in refluxing of forging materials is prevented through reverse filling formation; and (2) the contacted point of the section mutation eccentric part of the blank with a lower mold of the final forging mold is A; the contacted point of the main body part of the blank with the lower mold of the final forging mold is B; the highest point of the spherical column forming cavity of the lower mold of the final forging mold is C; the lowest point of a forming cavity of the main body part of the lower mold of the final forging mold is D; the height difference between the point A and the point B is a; the height difference between the point C and the point D is d; the height difference a is equal to the height difference b; the positioning is performed through the height difference; the blank is machined; and the final forging is prepared.