Influence of underwater turbulence on the performance of orbital angular momentum (OAM) communication system is investigated. The change of OAM communication system capacity with different environment parameters and transmission distances is presented. Results show that in underwater turbulence environment, the crosstalk between OAM beam modes is enhanced and channel capacity decreases with the increasing of transmission distance, dissipation rates of turbulent flow and temperature. When the transmission distance approaches 7 m, the channel capacity decreases to half of the original one. When the turbulence kinetic energy dissipation is close to 10－4m2/s3, the capacity decreases to 75% of the theoretical maximum. When the temperature dissipation rate is 10－8K2/s, the channel capacity reduces to half of the theoretical maximum. In underwater environment, salinity disturbance has a greater influence on the capacity performance than that of temperature disturbance.