A one-way retractable building formwork apparatus for mounting on a mounting surface of a substrate module, comprising: a first side bar configured to be disposed on one end of the mounting surface that extends along a lateral direction; a second side bar configured to be disposed on another end of the mounting surface that extends along the lateral direction and opposite to said first side bar along an extending direction transverse to the lateral direction; and a retractable unit configured to be disposed on the mounting surface and connected to said first side bar, said retractable unit being extendable along the extending direction to fixedly connect to said second side bar; whereby said retractable unit includes: a fixed plate assembly configured to be disposed on the mounting surface and including a first fixed plate that extends along the extending direction, that is configured to be disposed on one end of the mounting surface which extends along the extending direction, and that is fixed to one end of said first side bar, and a second fixed plate that extends along the extending direction, that is spaced apart from and that is located inwardly of said first fixed plate along the lateral direction; characterized in that said retractable unit includes at least one connecting bar assembly including a first connecting bar and a second connecting bar extending along the extending direction and located between said first fixed plate and said second fixed plate, said first connecting bar being slidably engaged with said first fixed plate, said second connecting bar being slidably engaged with said second fixed plate; and a slide bar assembly including at least one slide bar that extends along the extending direction and that is slidable between said first connecting bar and said second connecting bar to fixedly connect to said second side bar. a first side bar configured to be disposed on one end of the mounting surface that extends along a lateral direction; a second side bar configured to be disposed on another end of the mounting surface that extends along the lateral direction and opposite to said first side bar along an extending direction transverse to the lateral direction; and a retractable unit configured to be disposed on the mounting surface and connected to said first side bar, said retractable unit being extendable along the extending direction to fixedly connect to said second side bar; whereby said retractable unit includes: a fixed plate assembly configured to be disposed on the mounting surface and including a first fixed plate that extends along the extending direction, that is configured to be disposed on one end of the mounting surface which extends along the extending direction, and that is fixed to one end of said first side bar, and a second fixed plate that extends along the extending direction, that is spaced apart from and that is located inwardly of said first fixed plate along the lateral direction; characterized in that said retractable unit includes at least one connecting bar assembly including a first connecting bar and a second connecting bar extending along the extending direction and located between said first fixed plate and said second fixed plate, said first connecting bar being slidably engaged with said first fixed plate, said second connecting bar being slidably engaged with said second fixed plate; and a slide bar assembly including at least one slide bar that extends along the extending direction and that is slidable between said first connecting bar and said second connecting bar to fixedly connect to said second side bar. a fixed plate assembly configured to be disposed on the mounting surface and including a first fixed plate that extends along the extending direction, that is configured to be disposed on one end of the mounting surface which extends along the extending direction, and that is fixed to one end of said first side bar, and a second fixed plate that extends along the extending direction, that is spaced apart from and that is located inwardly of said first fixed plate along the lateral direction; characterized in that said retractable unit includes at least one connecting bar assembly including a first connecting bar and a second connecting bar extending along the extending direction and located between said first fixed plate and said second fixed plate, said first connecting bar being slidably engaged with said first fixed plate, said second connecting bar being slidably engaged with said second fixed plate; and a slide bar assembly including at least one slide bar that extends along the extending direction and that is slidable between said first connecting bar and said second connecting bar to fixedly connect to said second side bar.