Water-laser coupling transmission efficiency is the premise and efficiency guarantee of water-jet guided laser machinability. In order to study the law of water-laser coupling energy transmission in water-jet guided laser, and to obtain high laser power transmission efficiency and uniform laser power density distribution in water beam, by using ray tracing theory and physical optics propagation method, the simulation analysis of the focused beam characteristics at 1064nm and the speckle beam pattern after coupling were carried out. And the laser power transmission efficiency under different water beam length, the distribution of laser power density under different power, pressure and water beam length were tested and analyzed systematically. The results show that the power transmission efficiency of 1064nm laser increases with the decrease of the water beam length, and the power transmission efficiency can reach 63.6% when the water beam length is 20mm. The variation of laser power has the greatest influence on the distribution of laser power density in water beam. When the laser power is constant, the increase of water pressure is beneficial to the uniform distribution of laser power density in the stable length of water beam, and the decrease of coupled water beam length can improve laser transmission efficiency. The research results provide some guidance for improving the energy utilization rate of water guided laser.