The invention discloses a construction method of a foundation slab in subsurface structure engineering, which comprises the following steps: 1) cutting a pile head of a foundation engineering pile and finishing the pile head of the foundation engineering pile to designed elevation before the construction of the foundation slab; 2) adopting the pile head of the foundation engineering pile as a support point of a truss, forming trusses which are welded with the pile head of the foundation engineering pile respectively in the horizontal direction and the longitudinal direction, wherein all the trusses form a lattice type support frame, the bottom surface elevation and thickness of the support frame should meet thickness requirements on a foundation slab concrete protective layer, the section of each truss is in an inverted triangle shape, the bottom chord of the truss is one of pieces of rebar at the lower layer of the foundation slab, and the upper chords of the truss are two of pieces of rebar at the upper layer of the foundation slab; 3) forming the foundation slab rebar on the support frame; and 4) casting concrete and maintaining. The construction method of the foundation slab in subsurface structure engineering can protect a waterproof layer and ensure the thickness of the foundation slab concrete protective layer, enable the rebar at the upper layer and the rebar at the lower layer of the foundation slab to be bond stably and smoothly, and improve construction safety coefficient.