Abstract: Aiming at the problem that the output spectrum of narrow linewidth laser is narrow and difficult to be locked, the Fabry-Perot(F-P) cavity which has the unique narrow linewidth and high-definition features was used to do linewidth narrowing and the frequency locking. Based on the advantages of F-P cavity, the experiment plan was designed and a test platform was built, the external cavity optical feedback technology was employed to improve the accuracy of locked frequency. The transmission line was monitored under the sine wave modulation of F-P cavity with different laser piezoelectricity (PZT) sweep frequencies at four direct current(DC) voltages, then the demodulation was conducted and the locked frequency accuracy was tested. As a result, when using 73 V DC high voltage amplifier to sweep the narrow linewidth laser, the laser feedback locked frequency precision can be up to 1.5 MHz.