The second-order nonlinear optical material, 2,4-dinitrophenol (DNP) was synthesized. Measured results show that the space group of DNP is P212121 and the SHG efficiency of DNP is 5 times as large as that of KDP. The calculations of the second-order nonlinear optical polarizability β with CNDO/S-CI program reveal that the β is characteristic anisotropy, and the intramolecular charge transfer being responsible for the optical nonlinearity predominantly is from the hydroxy group to the nitro group in ortho position. The second-order nonlinear optical properties of 2,4-dinitrophenol (DNP), and the relationship between the microscopic and macroscopic nonlinearity are also analyzed. The size of DNP crystals is up to 21 mm×17 mm×7 mm.