Abstract: By using a sound speed measurement device composed of probe and CTS-3600 digital ultrasonic velocity measurement,ultrasonic propagation characteristic in near-critical CO₂ has been studied.Research results show that both ultrasonic speed and relative sound intensity are the function of system density in either near-critical CO₂ system or near-critical CO₂ with entrainer system.With pressure increasing,both of them increase.At the same time,with temperature increasing,both of them decrease.Ultrasonic velocity and relative sound intensity in the near-critical CO₂ with entrainer system are higher than those in near-critical CO₂ system,the main reason might be that addition of entrainer alcoholic increases system density.