Co-adsorption of formaldehyde (HCHO) with hydroxyl groups on both clean and hydroxylated TiO₂-B(100) surfaces with terminal and bridging hydroxyl groups is investigated by using first-principles method. The hydroxyl groups was found to affect chemisorption configurations of HCHO on both clean and hydroxylated TiO₂-B(100) surfaces. It indicates that bridging hydroxyl on most adsorption sites near to HCHO weakens adsorption of HCHO, while terminal hydroxyl on most adsorption sites facilitates it. Investigation of hydroxyls on HCHO in different periodicities shows that terminal hydroxyl usually has significantly facilitated on adsorption of H₂O in larger periodicity, while bridging hydroxyl does not have this trend. This unique mechanism provides a novel angle to understand interaction of HCHO with hydroxylated TiO₂ surface.