An optical frequency conversion method, apparatus, and device are provided. Micro-nano fibers and guiding fibers are cascaded, to change an optical frequency conversion manner from using a long micro-nano fiber as a frequency conversion medium to cascading a first quantity of shorter micro-nano fibers and a second quantity of guiding fibers to perform optical frequency conversion. A length of each micro-nano fiber is not greater than a coherence length of a fundamental-frequency pump light signal and a frequency-tripled light signal. The frequency-tripled light generated by cascaded micro-nano fibers is coherently superposed. A phase difference between frequency-tripled light components is controlled by adjusting incident power of the fundamental-frequency pump light, to achieve constructive interference, thereby significantly enhancing the frequency-tripled light signal and effectively improving the optical frequency conversion efficiency.