The invention discloses a preparation method of a variable gradient phenolic impregnated carbon ablator composite material (PICA). According to the preparation method, carbon felt treated via surface treatment is taken as a reinforcing material, boron modified phenolic resin is taken as a matrix material, layered preparation method is adopted, and the variable gradient PICA is prepared, wherein the density of the two side parts of the variable gradient PICA is low, and the density of the middle part of the variable gradient PICA is high. In the preparation method, acetone desizing, gas-liquid double effectiveness treatment, and coupling agent treatment are adopted for surface treatment of the carbon felt; the carbon felt obtained via surface treatment is impregnated into an absolute ethyl alcohol solution of boron modified phenolic resin of a certain mass concentration, and vacuum pressure impregnation is adopted so as to obtain PICA thin sheets of a certain thickness; the PICA thin sheets of different density are overlapped layer by layer, and are delivered into a muffle furnace for curing molding; and a series of performance characterization technologies are adopted for optimization of preparation technological parameters so as to obtain the variable gradient PICA material with the required density and gradient.