The invention provides a composite retaining structure of a foundation pit project and a construction method thereof, and relates to the technical field of foundation pit retaining structures. The composite retaining structure of the foundation pit project and the construction method thereof aim at solving the problems that after an underground diaphragm wall is constructed, deformation, pit bottom heaving and other unfavorable phenomena are generated due to the fact that reinforcing measures are not taken when a foundation pit is dug deeper. The construction method comprises the steps that cast-in-situ bored piles are additionally arranged on the inner side of the underground diaphragm wall, high-pressure jet grouting piles are arranged between the underground diaphragm wall and the cast-in-situ bored piles in a filling mode, the foundation pit is dug step by step, and ring beams and reinforced concrete supports are constructed layer by layer; notches are reserved in the upper edge of a foundation slab, and downwards-turned brackets are constructed on the lower edge of the foundation slab; and a post-cast strip is formed after the cast-in-situ bored piles and the high-pressure jet grouting piles above the bottom elevation of the notches are dismantled, and concrete at the post-cast strip is made to be fixedly bonded with the foundation slab into a whole. The composite retaining structure comprises the cast-in-situ bored piles on the inner side of the underground diaphragm wall and the foundation slab fixed at the top ends of the cast-in-situ bored piles and the top ends of the high-pressure jet grouting piles; the high-pressure jet grouting piles are arranged between the underground diaphragm wall and the cast-in-situ bored piles in the filling mode, and the downwards-turned brackets are further arranged on the edge of the bottom face of the foundation slab.