The invention relates to a thin-wall titanium alloy complex variable-cross-section pipe forming method and application and belongs to the technical field of material machining. A titanium alloy sheet is cut according to the size of an unfolded variable-cross-section special-shaped pipe, and a special-shaped titanium plate is obtained; the special-shaped titanium plate, a female die and an upper die are placed in diffusion bonding equipment, and after heating is conducted, a core die is completely pressed into the female die; a left die and a right die are fed, the sheet is closed to form the special-shaped pipe, an upper working platform is downwards pressed, heating is conducted till the diffusion welding process temperature is reached, then joint diffusion welding is conducted, cooling is conducted, and the thin-wall titanium alloy complex variable-cross-section pipe is taken and obtained. By means of the method, then high-quality titanium alloy variable-cross-section pipe which is very even in wall thickness, high in size accuracy and nearly invisible in weld joint can be obtained just through two processes and one-time heat circulation, and large-scale production can be achieved.