Axisymmetric drop shape analysis (ADSA) has been applied to study the interfacial interactions of CO2 and reservoir fluid systems at elevated pressures and high temperatures. Using ADSA for the pendant drop technique, it was possible to determine the interfacial tension (IFT) and visualize the interfacial interaction phenomenon of CO2/crude oil, CO2/brine, crude oil/brine, and crude oil/CO2-saturated brine systems under actual reservoir conditions. (1) For the CO2/crude oil system, there was a light component extraction and initial turbulent mixing when crude oil contacted CO2. The interfacial interaction between CO, and crude oil increased and the IFT of the CO2/crude oil system decreased as pressure increased. (2) For the CO2/brine system, the wettability between brine and needle surfaces was altered from hydrophilicity to hydrophobicity as pressure increased and CO2 changed from a gas state to a supercritical state. The IFT of the CO2/brine system decreased as pressure increased. (3) For the crude oil/CO2-saturated brine system, there were not interfacial interactions similar to those of the CO2/crude oil system. The interface of the crude oil/CO2-saturated brine system was as clear as that of crude oil/brine. The IFT of the crude oil/CO2-saturated brine system was slightly lower than that of crude oil/brine due to CO, dissolution. For the IFT of the four systems, that of CO2/crude oil was the lowest, while those of CO2/brine, crude oil/brine, and crude oil/CO2-saturated brine systems were similar.