Wood is a natural, organic material that cannot be exposed to high temperatures, fire, or heat, especially when wood or wood-based materials are used as construction elements. The fire-extinguishing composition of di-ammonium phosphate, ammonium sulfate, and ammonium bromide (DAB) was used to increase the fire-resistance of birch plywood. The effects of various parameters of diffusive impregnation of the veneer (temperature and concentration of impregnating solution, duration of impregnation) were investigated. Their dependence was linear for the temperature of impregnating solution, and logarithmic for concentration of impregnating solution and duration of impregnation. Increased retention of fire retardant improved plywood fire resistance. However, considering the quality of impregnation, energy costs, and plywood properties, the following parameters of wet veneer impregnation are recommended: a temperature of impregnating solution of 22 °C, concentration of 30%, and duration of impregnation of 8 min.