使用压缩蒸气模型，推导了超临界流体在超临界点附近区间的声速表达式，表达式揭示了声速和密度波动指数、等温压缩系数、定体摩尔热容量等参量的联系.在超临界点附近，二氧化碳流体的声速和密度波动指数呈减函数关系，密度波动指数越大，声速越小，在密度波动指数最小处，声速最大，此时，较小的密度波动会引起较大的声速波动.当压强逐渐增大并接近临界点时，定体热摩尔容量的迅速增大导致声速减小，当压强增大而远离临界点时，定体摩尔热容量的迅速减小导致声速增大.由表达式得到的计算值与由美国国家标准局提供的参考值符合较好. 超临界二氧化碳 /  声速 /  密度波动 /  定体摩尔热容量  Based on the condensing vapor model, the expression of sound speed close to critical point is deduced,which shows the relationship between the sound speed and the density fluctuating index and thermal capacity at constant volume. Close to critical point, the sound speed of liquid carbon dioxide is inversely proportional to the density fluctuation index. The bigger the density fluctuation index, the smaller the sound speed. The sound speed is maximal at minimal density fluctuation index, where small fluctuations of density induces large fluctuations of sound speed. When the pressure increases and approaches to critical point, the rapid increase of thermal capacity induces decrease of sound speed. When the pressure increases and departs from critical point, the rapid decrease of thermal capacity at constant volume induces increase of sound speed. The calculated values of sound speed from the expression are in good agreement with the reference values from NIST. Keywords: supercritical carbon dioxide /  sound speed /  density fluctuation index /  thermal capacity at constant volume  刘祥群, 刘宇, 凌艺铭, 雷久侯, 曹金祥, 李瑾, 钟育民, 谌明, 李艳华. 等离子体风洞中释放二氧化碳降低电子密度 . 物理学报, 2022, 71(14): 145202. doi: 10.7498/aps.71.20212353 李雪梅, 俞宇颖, 谭叶, 胡昌明, 张祖根, 蓝强, 傅秋卫, 景海华. Bi在固液混合相区的冲击参数测量及声速软化特性 . 物理学报, 2018, 67(4): 046401. doi: 10.7498/aps.67.20172166