The present invention relates to an automobile urea tank and a forming method thereof. The method includes the following steps: 1) producing an upper half casing and a lower half casing through an injection molding method; 2) after the upper half casing and the lower half casing are formed, assembling built-in parts of the upper half casing and the lower half casing; 3) welding the assembled upper half casing and the assembled lower half casing to form a whole; and 4) assembling components on surfaces of the upper half casing and the lower half casing. This method can improve design freedom of built-in elements of the tank body, and can save a lot of raw materials; the structure is simple and firm; moreover, the production efficiency is improved, and the production cycle is shortened.