Provided is a method for preparing hydrophobic monolithic silica aerogel, comprising dipping monolithic wet silica gel obtained by using an alkoxide precursor into an alkylsilane solution as a dipping solution to perform hydrophobitization of the surface and inner part of the monolithic wet silica gel by a dipping process. The method is economical by virtue of the use of a small amount of alkylsilane compound and imparts hydrophobic property to monolithic silica aerogel simply in a cost efficient and time efficient manner. In addition, the method reduces shrinkage of hydrophobic monolithic silica aerogel, enables production of hydrophobic monolithic silica aerogel in a translucent form, and allows the hydrophobic monolithic silica aerogel to maintain low heat conductivity similar to the heat conductivity of hydrophilic silica aerogel. The hydrophobic monolithic silica aerogel may be used directly as a heat insulating panel by virtue of excellent hydrophobic property and heat insulating property.