The invention discloses a temporary steel trestle constructing method and a trestle and relates to the field of bridge construction. The temporary steel trestle constructing method sequentially comprises the following steps of prefabricating standardized structural elements, prefabricating synthetic-wood bridge deck slab standardized structural elements and splicing bridge deck layers. The trestle comprises a supporting structure, a main beam arranged above the supporting structure, a beret frame arranged above the main beam and a synthetic-wood bridge deck layer arranged above the beret frame. The synthetic-wood bridge deck layer is formed by a plurality of synthetic-wood bridge deck slabs which are horizontally and sequentially spliced. Each synthetic-wood bridge deck slab is formed by a plurality of synthetic-wood primary beams, a plurality of synthetic-wood secondary beams, a plurality of synthetic-wood clamping beams and a plurality of synthetic-wood face boards which are integrally spliced. The temporary steel trestle constructing method is convenient, quick and reliable to construct, achieves the purpose of environmental friendliness, is economical and is low in construction cost. The trestle erected through the constructing method is high in strength, better in reliability, better in mechanical performance, better in corrosion resistance and better in fire resistance. Furthermore, components of the trestles can be used repeatedly.