The invention relates to a construction method for an assembled foundation pit support structure. A prefabricated reinforced concrete pile and a prefabricated box type component are mainly adopted. The prefabricated box type component comprises at least one standard piece. Trapezoid tenons 11 and trapezoid mortises 12 are formed in transverse and longitudinal connector positions of the standard pieces in a staggered manner. The prefabricated reinforced concrete pile comprises a front part 41, a rear part 42 and a pile head 43, wherein trapezoid tenons 412 and trapezoid mortises 413 are formed in the two sides of the front part in a staggered manner, and the tenons and the mortises of the prefabricated reinforced concrete pile and the tenons and the mortises of the prefabricated box type component are matched in size. A clamp support 421 used for supporting a prefabricated box type component located at the adjacent position is arranged at the position, close to the front part 41, of the rear part 42. After the whole support structure is spliced, compaction grouting is conducted on the position of a last anti-seepage grouting hole where the corresponding standard piece is arranged, and the anti-seepage grouting holes formed by the standard pieces and a pile body are fully filled with pulp from bottom to top. When it is observed that the pulp overflows out of an anti-seepage grouting hole channel of the top-layer standard piece, grouting is stopped.