The invention relates to a prefabricated steel trestle construction method for a foundation pit. The prefabricated steel trestle construction method comprises the following construction steps: cast-in-place pile construction, steel latticework welding, steel latticed column and cast-in-place pile installation, separate type prefabricated fastening steel plate installation, doughnut-shaped steel plate installation, diagonal brace rod installation, steel latticed column top steel plate installation, double-H profile steel installation, bailey frame assembly, basic unit bailey truss installation, trestle lifting plate erection, trestle slope plate erection, central trestle plate erection, concrete support and steel latticed column connection, thick steel plate installation, cross bracing system installation, steel plate installation, guardrail installation, foundation pit excavation, thick steel plate dismantling, cross bracing system dismantling, guardrail installation, trestle lifting plate dismantling, trestle slope plate dismantling, central trestle plate dismantling and steel latticed column dismantling. According to the prefabricated steel trestle construction method for the foundation pit, the problems that the construction period of foundation pit excavation is long, the construction cost is high and the construction difficulty is high are solved, the construction efficiency is improved and better technical and economic benefits can be obtained if the construction method is applied to practical engineering.