The invention discloses a preparation method of a P-layer silicon epitaxial wafer on a P++ substrate, and belongs to the technical field of silicon epitaxial wafers. The preparation method comprises the steps as follows: (1) selecting a P+<111>/Boron single-face polished section; (2) performing HCl polishing; (3) sweeping; and (4) performing double-layered epitaxial growth: adopting high-purity SiHCl3 deposition, growing a resistivity gradient layer, and then growing a resistivity doped layer. The preparation method adopts a secondary growth process to improve the uniformity of the resistivity of an epitaxial layer; the concentration gradient layer is grown at a low speed, so that formation of misfit dislocation of the epitaxial layer is reduced effectively, the stability and the repeatability of epitaxial electrical properties are improved, and the reliability and the yield of devices are guaranteed.