As many properties of lignin are relied on moisture content, water adsorption is very important for its product performance. To better understand water adsorption mechanism of lignin, the molecular interactions between adsorbed water and lignin was studied using micro-FTIR spectroscopy. Spectra of lignin were collected at various relative humidity (RH) levels from low to high. From qualitative analysis of these spectra and corresponding difference spectra, water adsorption sites of lignin was recognized, and the spectral ranges closely related to water adsorption were identified. Further, from component peak analysis of the identified spectral region, three component peaks were attributed to three types of bound water separately. Based on the change of bound water vs. RH, the moisture adsorption process of lignin could be separated to three parts. Moreover, molecular association of adsorbed water with lignin was all demonstrated in these three parts.