Photocatalytic activation of peroxymonosulfate (PMS) on TiO2 is promising for wastewater treatment, but it is unknown which crystal phase, anatase or rutile, is more suitable for this purpose. Herein, mesoporous TiO2 materials made of anatase, rutile or their homojunctions with tunable ratios are synthesized and their photocatalytic PMS activation behaviors are unraveled. PMS activation depends on photo-generated electrons. Anatase is more effective than rutile in activating PMS to produce SO4•‾/HO•. The homojunction with 90% of anatase promotes charge separation and exhibits the best performance to degrade carbamazepine via photocatalytic PMS activation. The optimized homojunction shows superior activity, excellent stability and high tolerance to different anions, water matrices and pH values. Theoretical calculations and spectroscopic results reveal PMS can be more easily adsorbed and dissociated on rutile than anatase, but the energy barrier for desorption of the dissociated products from rutile is much higher, accounting for the poor PMS activation on rutile.