Combined with the solar spectrum, the photon absorption coefficient of the β-FeSi2 thin film has been analyzed, and then the thickness of the absorption layer of β-FeSi2 thin film solar cell has been analyzed and theoretically calculated. The results show that, under the condition of the β-FeSi2 thin film with high quality, when the optical absorption efficiency of the solar energy radiation reaches 90%, the absorption layer thickness of the β-FeSi2 thin film solar cell is at least 200 nm and the best thickness range is from 200 to 250 nm. The calculation result has been verified by some relative experimental studies. At the same time, the formula for the relation between the absorption layer thickness of the β-FeSi2 thin film solar cell and the solar photon wavelength has been obtained.