When a vortex beam propagates in a turbulent medium, the phase will be distorted by the influence of turbulent motion. In this paper, a point pixel phase extraction method based on heterodyne interference is proposed. By collecting the relative phase of each point's interference fringes in a period, the phase distribution of a vortex beam is reconstructed after its turbulent transmission under water. The random phase screen is generated by the power spectrum inversion method, and the transmission simulation of the vortex beams under different turbulence is completed. The result shows that the distortion increases with the increase of turbulence intensity. An experimental device was built to realize the phase measurement of vortex beams in underwater environments. The experimental results show that when a vortex beam is transmitted in turbulent water, its complex amplitude distribution undergoes a relatively complicated transmission process, and its intensity distribution and phase are distorted. The method proposed in this paper can accurately measure the complex phase distribution of a vortex beam and effectively identify its topological charges.