The embodiment of the invention relates to a tunable laser frequency expansion method. The tunable laser frequency expansion method comprises the following steps of 101, generating fundamental frequency light with a certain wavelength Lambda, and irradiating the fundamental frequency light to a frequency doubling crystal for frequency doubling at an angle Theta1, wherein frequency doubling is I-type phase matching conforming to a phase matching condition; 102, irradiating the fundamental frequency light and frequency doubling light which are emitted from the frequency doubling crystal to a long right-angle edge of a Pellin-Broca prism at an angle Phi1; 103, enabling the fundamental frequency light and the frequency doubling light to be refracted in the Pellin-Broca prism and irradiate fromthe Pellin-Broca prism along different directions, wherein the frequency doubling light irradiates along a direction perpendicular to an original incident direction after irradiation from the Pellin-Broca prism; and 104, irradiating the frequency doubling light to a 45-degree total-reflective mirror after irradiation from the Pellin-Broca prism, wherein the frequency doubling light is output along an original fundamental frequency light direction by the total-reflective mirror. By employing the Pellin-Broca prism, the frequency doubling light and the fundamental frequency light can be effectively separated.