The invention provides a one-time two-drum forming machine for a car tyre and a method thereof. A basic structure of a two-drum and three-station forming machine is provided, and by pre-preparing a wire loop and a belt ply-tread assembly, an axially movable forming drum bears a tyre assembly to enter a forming and rolling station, thereby effectively optimizing an integral manufacture process, further shortening time for preparing a single green tyre, and raising production efficiency. The forming machine comprises a main machine and a feed rack, wherein arranged along a main shaft center line along a main machine is: a belt ply drum fitting station lying on one side of the of a belt ply drum; a main feed rack fitting station lying on one side of the forming drum; a forming and rolling station lying on one side of a combination press roller, wherein a prepared belt ply-cap ply strip-tread assembly and a tyre side-lining layer-tyre curtain is conveyed to the main feed rack fitting station to be combined to the wire loop, to perform a radially-building and rolling operation process for the green tyre; and the forming and rolling station, lying between the belt ply drum fitting station and the forming and rolling station.