The thickness measurement sensor of alkali metal gas chamber based on spectral confocal is a non-contact thickness measurement sensor, which can obtain the thickness value of transparent material by directly measuring the wavelength information of light wave. The dispersive objective is the core component of the thickness measurement sensor. The linearity and dispersion range of the dispersive objective lens determine the accuracy and resolution of the thickness measurement sensor. The principle of thickness measurement sensor based on spectral confocal is introduced. The relationship between wavelength information and axial dispersion range is analyzed. The dispersive objective lens is designed by using ZEAMX optical design software, and optimizes it with multiple structures. The axial dispersion range of 0.801mm is achieved in the wavelength range of 420~620nm. The linear fitting of wavelength and axial dispersion by least square method is 0.9975. With a high resolution spectrometer of 0.001nm, the measurement accuracy of the sensor can reach nanometer level, which can meet the needs of high-precision measurement of alkali metal gas chamber wall thickness.