The invention provides a foundation slab post-cast strip waterproof steel plate combined structure and a bracing method thereof. The foundation slab post-cast strip waterproof steel plate combined structure comprises a waterproof steel plate, wherein upper and lower wire meshes are respectively arranged at longitudinal center of upper and lower parts of the waterproof steel plate; reinforcing bar brackets are respectively welded on two sides under the waterproof steel plate; and a distance between two adjacent reinforcing bar brackets is not more than 500mm. The bracing method of the foundation slab post-cast strip waterproof steel plate combined structure comprises the following steps: constructing a cushion layer; constructing a mortar bar; welding the waterproof steel plate combined structure; binding a base plate and the reinforcing bars thereon; bracing the waterproof steel plate combined structure; fixing a multiplayer plate combined structure; and pouring the base plate concrete and the post-cast strip concrete. The foundation slab post-cast strip waterproof steel plate combined structure has an excellent anti-seepage effect, can reliably realize a waterproof function and can prevent the phenomena of waterproof steel plate shifting and impurities falling into the post-cast strip caused by the overflow of the concrete during the process of pouring the concrete.