A chlorine doped titaniumdioxide photocatalyst was synthesized by a hydrothermal process using silica gel as a carrier, Ti(SO4)2 as a Ti source, and NaCl as a Cl source.We discuss factors such as the doping ratio of chlorine to titanium, the calcination temperature, and the ratio of titanium to silicon, all of which influence the photocatalytic activity of the sample.We found that the best chlorine to titaniumratio (nCl/nTi) was 15%, the best calcination temperature was 750 ℃, and the best ratio of titaniumto silicon (nTi/nSi) was 3:1. The BET surface area of the chlorine doped titanium dioxide supported on SiO2 (CTS) was 60.8 m2 ·g-1. No rutile phase of TiO2 was formed in the CTS after calcination at high temperature indicating that the presence of silica inhibited the phase transformation of TiO2 from anatase phase to rutile phase. It also influences the structural and textural properties of the chlorine doped titanium dioxide (CT). A phenol degradation test showed that the photocatalytic activity of the CTS was better than that of the CT although the TiO2 concentration in the CTS was 0.7740 g·g-1, which was less than that of the CT.