Three-frequency ratio technology is important for sodium Doppler lidar to detect the temperature and wind field in the mesosphere. To realize this technology, three emitted lasers with 589 nm wavelength (f₊, f₀, f_-) should be frequency stable, the frequency switching speed should be fast and the two frequencies (f₊, f_-) should be frequency locked to 630 MHz relative to the central frequency (f₀). In order to meet the above technical requirements, a scheme based on optical phase-locked loop (OPLL) is proposed to realize three-frequency ratio technology. The OPLL is used to realize frequency offset locking. The frequency offset locking range is ± (200~2500) MHz, the minimum step is 200 kHz and the jitter of the beat signal is within ±50 Hz. By setting the frequency shift value and changing the polarity of the error signal, the wavelength of the slave laser is constantly changed, and the three-wavelength output is realized. The PID (proportional, integral, derivative) circuit is optimized to improve the stability of the beat signal, reduce the frequency switching time and the frequency cutting time is less than 10 ms. The experimental results show that the OPLL system can realize fast switching and locking of three frequencies and meet the technical requirements of three-frequency ratio technology.