The invention discloses a determining method for the optimum thickness of a guard post reserved protection layer in deep foundation pit blasting construction. The number of to-be-optimized thicknesses of the optimum thickness delta of the reserved protection layer (2) is set to be n+1 within the optimal range of the optimum thickness delta of the reserved protection layer (2); then according to a blasthole (3), explosive (4), the n+1 to-be-optimized thicknesses of the optimum thickness delta of the reserved protection layer (2) and a finite element model of a guard post (1), the pressure peak Fci and the tension peak Fti applied to the guard post (1) under the action of the blast load are obtained sequentially; and then the pressure peak Fci is compared with the compression bearing capacity [Fc] of the guard post (1), the tension peak Fti is compared with the tension capacity [Ft] of the guard post (1), and the corresponding i to-be-optimized thickness delta i which first meets the requirements that the pressure peak Fci is smaller than the compression bearing capacity [Fc] and the tension peak Fti is smaller than the tension capacity [Ft] is the optimum thickness delta of the reserved protection layer (2). The determining method can meet the actual bearing requirement and can also ensure the excavation safety of a deep foundation pit, high efficiency and maximum benefit.