CO2 sequestration has been considered to be one of the most straightforward carbon management strategies for industrial CO2 emission. Monitoring of the CO2 injection process is one of the best ways to make sure the safety storage but is also a major challenge in CO2 geological sequestration. Previous field and laboratory researches have shown that seismic methods are among the most promising monitoring methods because of the obvious reduction in P-wave velocities caused by CO2 injection. However, as CO2 injection continues, the P-wave velocity becomes increasingly insensitive according to the pilot projects when CO2 saturation is higher than 20-40%. Therefore, the conventional seismic method needs improvement or replacement to solve its limitations. In this study, P-wave velocity and amplitude responses to supercritical CO2 injection in brine-saturated core samples from Jilin oilfield were tested using core displacement and an ultrasonic detection integrated system. Results showed that neither the P-wave velocity nor amplitude could simply be used to monitor the CO2 injection process because of the insensitive or nonmonotonous response. Consequently, a new index was established by synthetically considering these two parameters to invert and monitor the CO2 process, which can be thought of as a newer and more effective assessment criterion for the seismic method.