This study investigates the adsorption and reactions of boron trichloride and its fragments (BClx) on the TiO2 anatase (101) and rutile (110) surfaces by first-principles calculations. The results show that the possible absorbates on the TiO2 anatase and rutile surfaces are very similar. The single- and double-site adsorption configurations are found for both anatase and rutile surfaces. The particular adsorbate feature on the anatase surface is its in-plane double-site adsorption by Ti and O from its sawtooth surface. The potential energy surface shows that BCl3 can be adsorbed on the O site for both the anantase and rutile surfaces and the most of the BClx reaction on both anatase and rutile surfaces are endothermic, except for the dissociative reaction on the rutile surface. The energy levels of the BClx reactions between the anatase and rutile surfaces show that the rutile surface has lower energy levels than those of anatase surface. This result reveals that the BClx dissociative adsorption more easily occurs on rutile surface than on anatase surface.