The invention provides a launch vehicle low-temperature composite storage tank with a triangular sandwich wall. The storage tank comprises a front bottom cover, a front sealing head, a tank cylinder section inner wall, a tank cylinder section outer wall, a rear sealing head, a rear bottom cover and a rear bottom tank housing and is characterized in that a triangular sandwich wall structure comprising reinforcing ribs is arranged between the tank cylinder section outer wall and the tank cylinder section inner wall; and Y ring triangular sandwich wall connecting structures are formed at connecting parts between the front sealing head and the tank cylinder section inner wall, between the front sealing head and the tank cylinder section outer wall, between the rear sealing head and the tank cylinder section inner wall and between the rear sealing head and the tank cylinder section outer wall. The invention further discloses a processing method of the storage tank, which employs an integral co-bonding forming technology of fiber winding forming combined with fiber laying forming. According to the storage tank, the triangular sandwich wall structures are arranged at the high shear stress connecting parts between the sealing heads and a tank cylinder section; the shear strength of the connecting parts is improved; and the storage tank has the effects that the axial carrying capacity of the storage tank is improved, the structure assembly complexity is reduced, and the production efficiency is improved.