The hydrophilicity of the normal decoction pieces (NDP) of Indigo Naturalis is not good, therefore, it is not suit for decoctions.  In this paper, powder modification technology is used and some NDP and alcohol are ground together in the vibromill to prepare the hydrophilic decoction pieces (HDP) of Indigo Naturalis.  Initially, the properties of NDP, ultrafine decoction pieces (UDP) and HDP are compared, the hydrophilicity of UDP was promoted slightly, that of HDP is promoted dramatically.  Then, three batches of Indigo Naturalis are prepared to HDP separately, but there is no obvious difference in the contact angle.  Furthermore, the size distribution, surface area and micro-shape of HDP are bigger than that of UDP and smaller than NDP.  The contents of indigo and indirubin in three decoction pieces are the same, as well as the species of inorganic substance, although there is a little difference in the proportion of five inorganic substances.  The fact suggests the change of physical state and the qualitative and quantitative change of organism and inorganic substances are not the main factors to influence the hydrophilicity.  In addition, hydroxyl, methylene and methyl can be identified at the wavenumber of 3 356 cm⁻¹ and 1 461 cm^{−1 in} infrared spectrum; the content of alcohol in HDP is 0.67% measured by gas chromatogram.  The stability of HDP in the heating condition is studied, the fact suggests the hydrophilic effect of HDP at 40 ℃ is relatively stable.  All above research suggests that the alcohol is the main factor to influence the hydrophilicity and maybe the intermolecular force which fixed alcohol molecule on the surface of Indigo Naturalis is the basic principle to produce the hydrophilicity.