The invention discloses a construction method for a deep buried tunnel crossing a stratum with a soft upper layer and a hard lower layer. The construction method comprises the steps that the boundarybetween a soft stratum and a hard stratum is defined; the stratum is sectioned; a vault on the upper portion of a middle soft is subjected to advance strengthening; the middle soft stratum area is excavated; after the middle soft stratum area is excavated, a temporary support is constructed at an excavated area, and an arch bracket is erected; soft strata on the two sides are subjected to transverse advance strengthening; the temporary support is removed, the soft strata on the two sides are excavated, a side wall on the excavated area is sprayed with concrete, and a second annular arch bracket is erected; hard rock stratum areas on the two sides are excavated, after the excavation, a side wall on the excavated area is sprayed with concrete, and a third arch bracket is erected; a gate-typearch bracket is removed, and a middle hard rock stratum is exploded; a inverted arch and secondary lining are constructed; and the steps are repeated until tunnel excavation is completed. According to the construction method for a deep buried tunnel crossing the stratum with the soft upper layer and the hard lower layer, the stability of surrounding rocks is ensured, the settlement of the vault is avoided, the construction safety is ensured, and the construction efficiency is improved.