In order to provide a theoretical basis for the development of CO2 flooding in horizontal wells in oil reservoirs，a three-zone composite horizontal well model for CO2 flooding was proposed. A series of mathematical and physical methods such as Laplace transformation，orthogonal transformation，and numerical inversion were adopted to solve this model. According to the pressure response characteristics，the well testing curves were divided into nine stages including the wellbore storage stage，skin effect stage，early radial flow stage，linear flow stage，plane radial flow of CO2 zone，transfer stage of CO2 zone to transition zone，the radial flow in transition zone，the transfer stage from the transition zone to the crude oil zone and the radial flow in the crude oil zone. Sensitivity analysis was conducted on several influential factors，and results show that when the radius of the CO2 zone and transition zone become larger，the dimensionless bottomhole pressure difference will decrease，which is favorable for CO2 injection；the larger the storability ratio is，the shorter the flow time in the transition zone will be；the decrease of the wellbore storage coefficient and skin factor is more favorable for the displacement process；the greater the mobility ratio is，the longer the flow time in the transition zone will last.