The X-ray Photoelectron Spectroscopy (XPS) technique can provide information about the chemical states of various elements on the sample surface and the peak intensities and positions of their spectra. The thickness of thin films can be calculated by utilizing this information and the equation for photoelectron signals. This paper introduces three methods for handling the equation of photoelectron signals: direct solving method, substrate-ratio method, and angle-ratio method. Their derivation processes are analyzed, and the results indicate that these three methods have different accuracies and applicability ranges. The direct-solving method has the widest applicability but the lowest accuracy. The substrate-ratio method has the most limited applicability range, being able to calculate the thickness of thin films only on substrates of infinite thickness, and the effective attenuation length λ of the composition of film and substrate composition needs to be similar. However, it is least affected by instrumental and carbon contamination errors, thus having the highest calculation accuracy. The angle-ratio method has a moderate applicability range and calculation accuracy, and it can be used without considering the limitation of the substrate layer. However, the variation in the emission angle θ significantly affects the calculation accuracy. Researchers can consider the above factors comprehensively when calculating the thickness of thin films using XPS data.