Abstract: A hot model of a gas-solid circulating fluid downer reactor was built to study the deep catalytic cracking process with DAO feedstock and commercial equilibrium CHP-1 catalyst. The influence of reaction temperature and catalyst to oil ratio(CTO) on product′s distribution was examined. The experiments showed, with the increase of reaction temperature, light olefin yields increased at first to a maximum and then decreased. However, the light olefin yields increased with the increase of CTO. Owing to the short residence time, more uniform gas-solid flow and reduced backmixing of downer reactor, it can obtain more object products and the dry gas yield can be inhibited. The experiment results indicate that the downer reactor is suitable for deep catalytic cracking of heavy oil which needs high severity reaction condition.