To improve the hydrophobicity of wood, utilizing a silica/wax composite emulsion (PPW@SiO2), which was prepared by maleic anhydride-grafted polypropylene wax (PPW) and nanosilica, served as a hydrophobic agent. The hydrophobicity modifying process of fast-growing poplar was carried out by heating permeation and hot pressing to prepare in-depth hydrophobic wood. The results show that the hydrophobic agent can fill the pores of poplar partly and form a two-dimensional, “petal-like”, rough hydrophobic structure in the pores. The water contact angle (WCA) on the surface of modified poplar can reach 140.3° when the amount of SiO2 is 1%. Even after surface grinding of 3 mm, the WCA is still higher than 130°, indicating the hydrophobic wood has a good performance of in-depth hydrophobicity. Furthermore, the surface of hydrophobic wood presents an exceptional self-cleaning function, and the physical and mechanical properties are also improved. Compared with a reference sample, the density, surface hardness, modulus of elasticity, compressive strength, and modulus of rupture of hydrophobic wood are increased by 66.67%, 53.16%, 55.60%, 49.50%, and 40.87%, respectively, and the water absorption in hydrophobic wood is also significantly decreased.