Laser synthesis aims at synthesising a number of discrete laser beams into a coaxial beam to achieve higher output power which is several times of that of a single laser to achieve the higher output power. A novel hollow beam convertor is designed, which can be used in N (N≥3) way coaxial laser beam synthesis, and the numerical calculation and experimental study are done. Using this technology in the three-beam synthesis, laser synthesis efficiency is 89%, the time jitter is no more than 2 ns, laser pulse-width reaches about 10 ns (FWHM), and laser beam divergence angle is no more than 2 mrad.