By using a self-reference transfer oscillator method, two individual 1560 nm lasers with about 1.2 GHz frequency difference were phase locked to a 729 nm ultra-stable laser at two preset ratios. By measuring the beat frequency of the two 1560 nm lasers, fractional instabilities of $2\times {10}^{-17}$ at 1 s and $2\times {10}^{-20}$ at 10,000 s averaging time were obtained, and the relative offset compared with the theoretical value was $4.2\times {10}^{-21}\pm 4.5\times {10}^{-20}$. The frequency ratio of them was evaluated to a level of $1.3\times {10}^{-20}$ in one day’s data acquisition. This work was a preparation for remote comparison of optical clocks through optical fiber links. The technique can also be used to synthesize ultra-stable lasers at other wavelengths.