How to choose or develop a suitable catalyst to promote hydrogen production from supercritical water gasification of dewatered sewage sludge has become a essential technology to realize of technology resource utilization. To study the catalytic effect and mechanism of aluminium chloride (AlCl3), in the present study, AlCl3 was selected as catalyst, sewage sludge was selected as an object, intermittent high temperature and high pressure reactor was adopted. Supercritical water catalytic gasification experiment is accomplished under the conditions of 400 °C, 24 Mpa and 30 min. The influence of AlCl3 on hydrogen production and key product from supercritical water gasification of dewatered sewage sludge was analysed , and the catalytic mechanism was discussed. Results show that hydrogen production from supercritical water gasification of sewage sludge was significantly promoted by adding AlCl3. The hydrogen yield reached 11.52 mol/kg OM with 6 wt% AlCl3 added nearly 43 times higher than that of no AlCl3 added. Phenols polymerised from micromolecules was encouraged and the coke was inhibited when AlCl3 was added. The catalytic mechanism is the hydrolysis reaction of AlCl3, which can forming HCl and Al2(OH)3. HCl acted as an acid hydrolysis agent that facilitate carbohydrate hydrolysis to micromolecules under subcritical condition further gasificate to produce hydrogen under supercritical condition. Al2(OH)3 acted as an alkaline compound catalyzer that promote the water-gas shift reaction and improve the hydrogen yield. The above principles work together to promote supercritical water gasification of dewatered sewage sludge to produce hydrogen.