The invention belongs to the field of aeronautical manufacturing engineering/aircraft assembly and provides an equal-thickness moulding surface splitting casting superplastic forming/diffusion connection forming mould and a preparation method thereof. The superplastic forming/diffusion connection forming mould comprises two parts of an upper mould body and a lower mould body; the upper mould body and the lower mould body are composed of a moulding surface part and a bracket part correspondingly; and the four parts are machined correspondingly. Guiding pin mounting plates are located in the middles of the two opposite side faces of the upper and lower mould moulding surfaces; hanging columns are arranged on the two sides of the guiding pin mounting plates; temperature measuring holes and air feeding/discharging holes (9) are formed in the side faces of the upper and lower mould moulding surfaces; and lightening grooves are formed in the upper surface of an upper mould bracket and penetrate through the whole bracket part. According to the superplastic forming/diffusion connection forming mould and the preparation method thereof, accuracy of the dimension and moulding surface can be kept under the high temperature and high pressure conditions, the manufacturing precision of superplastic forming/diffusion connection parts is improved, thus the shapes of the superplastic forming/diffusion connection parts meet the assembly requirements, the service life of the mould is prolonged, and the production cost is reduced.