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.