The invention discloses a cold-formed thin-walled steel web combined PC box girder, comprising an upper flange, a lower flange and two truss type cold-formed steel combined webs. According to the invention, the web and the lower flange are both formed by combining the cold-formed thin-walled steel, the dead weight of the girder body is reduced, and span breakthrough is achieved; the web and the lower flange are in a truss form, so the overall stability of the structure can be improved, and the problem of poor local stability of the cold-formed thin-walled steel is solved; the webs are made of the cold-formed thin-walled steel, so the problem that webs of common prestressed concrete beam bridges are prone to cracking is fundamentally solved; and factory production of the cold-formed thin-walled steel and a prefabricated laminated slab can effectively accelerate the construction progress and improve the quality of components. The box girder of the invention has the advantages of being high in assembly speed, high in standardization degree, low in manufacturing cost, light in self-weight and excellent in bending resistance and crack resistance, can effectively solve the problems that a traditional concrete box girder component is complex in construction, high in manufacturing cost, large in self-weight, prone to web cracking and the like, and is suitable for bridge structure girders or main and secondary beams and other components of a building structure.