Abstract: Based on the nonlinear optics, the theoretical basis of 589 nm laser frequency-summed by 1 064 nm and 1 319 nm laser was analyzed. Phase-matching factor and optimum length of lithium triborate (LBO) crystal were numerically simulated and the corresponding effective sum-frequency coefficient was figured out on this analytic solution. The calculated effective sum-frequency coefficient is 0.952×10-12 m/V at phase matching angles of 90° and 3.39°. A simple and high-effective experimental scheme was designed and Nd:YAG laser pumped by diode output 1 064 nm and 1 319 nm laser, which were summed by pulse synchronization adjustment to generate 589 nm quasi-continuous yellow laser with an average power of 500 mW, the beam quality factor M2 is about 2.1 and the frequency-summed efficiency is 18%.