Abstract: The Mao?8 block in Zhagan sag is characterized by ordinary heavy oil reservoir of middle?deep strata, medium?low permeability and low temperature and pressure, based on which the experiments of solubility and viscosity reduction between CO2 and Mao ?8 block crude oil are conducted. The pilot tests of 10 horizontal wells are conducted using CO2 and viscosity breaker compound thermal recovery echnology, and the factors influencing the thermal recovery effects are analyzed. The results show that horizontal interval length, drilling encounter rate, reservoir physical property and steam injection quality are the main factors