The laser and water-jet coupling technology is the key to realize water-jet guided laser machining. In order to improve the coupling accuracy, a new method was put forward based on off-axis optical system. The system was mainly composed of two convex lens, one for adjusting the axial position of laser beam waist, another lens for adjusting the radial position. The method can effectively improve the resolution of the coupling device. The theoretical analysis of the method was carried out, and an experimental apparatus was designed according to the analysis. The experimental results show that this method increases the radial resolution of coupling device by 5 times, which is consistent with the design. This method has been applied successfully to couple the laser into the nozzle of 100 μm diameter, and it is also an innovative application of off-axis optical systems.