Optical spectrum analyzer (OSA) is an important test and analysis instrument in the field of optical communication. It is mainly used for testing the spectral properties of the light sources, fibers and optical elements. On the basis of introducing the basic principle of OSA, the elaborate calibration method of the wavelength is studied. Firstly, the relationship between the wavelengths and the samples as well as the detection band is analyzed. Then the tunable laser，the wavemeter and the least square method are used to calibrate several characteristic wavelengths. Finally, the verification experiments are made by using the near.infrared spectrum test system. After the calibration, the accuracy of the wavelengths is controlled within ±0.3 nm，which can meet the requirement of the specifications.