A building formwork plastic framework with an adjustable two-dimensional size is characterized in that the plastic framework is a rectangular framework structure formed by two L-shaped high impact resistance plastic assemblies, and a groove matched with edges of a formwork panel is formed in the inner side of the plastic framework; the L-shaped plastic assemblies are in a double tenon connection by ninety degrees, and a tenon head and a tenon groove at the ends are riveted and fixed through vertical penetrating rivets. The building formwork plastic framework is mainly made of the high impact resistance plastic materials and is formed in a hot machining mode, so that damage to the plastic formwork caused by throwing, collision and the like in the using process is effectively relived, the plastic formwork can be protected, the formwork can be moved conveniently to accelerate project progress, and construction cost is reduced; the formworktemplate can be cut along with the formwork size, the application range is wide, and scale production can be achieved; good elasticity is achieved, and thus slurry leakage in the using process of an ordinary formwork can be avoided. Moreover, the building formwork plastic framework is mainly made of the high impact resistance plastic materials and can be recycled and repeatedly used, and thus the building formwork plastic framework is an environment-friendly product.