The invention provides a mould and method for preforming a lateral tee upset part and a forming technique of a 45-degree lateral tee. The mould comprises a blank forming barrel with a cavity, an upsetting rod for upsetting an upset blank in the cavity, a base which is arranged in the blank forming barrel and at the bottom of the cavity and a punching bottom arranged in the base; and an opening communicated with the cavity is formed in the upper portion of the blank forming barrel, a base plane portion right facing the opening is arranged on the base, and the lower end of the upsetting rod extends into the cavity through the opening. When the step of using the mould to preform the 45-degree lateral tee upset part is finished, the distance between the bottom end of the upsetting rod and the plane portion of the base is larger than or equal to 640mm and smaller than or equal to 660mm. Thus, the height of the left side is fundamentally identical with the height of the right side of the final formed part of the 45-degree lateral tee, the casting defects of folding and cross flowing are overcome, and the final forming effect of the 45-degree lateral tee is better; and the machining allowance is small, and materials are saved.