X-ray photoelectron spectroscopy (XPS) has been widely employed in scientific research and engineering technology. The test data typically require handling for peak fitting; consequently, an accurate peak position in binding energy and the relative contents of the surface species are required. However, a serious human factor emerges during the process of XPS peak fitting, leading to distinct mistakes in the fitting results. The reason for the fitting errors is primarily that no constraints or incorrect constraints were applied to the fitting parameters of the XPS signals. There are four essential elements in XPS photoelectron signals: the separation distance and intensity ratio of spin-orbit splitting, the shape and symmetry of XPS signals, and the full width at half-maximum (FWHM) of XPS signals. In this text, the physical natures of these four essential elements are elucidated, and a reasonable suggestion is proposed for the setting of parameters in XPS peak fitting. In the process of XPS peak fitting, it is not simply a matter of mathematical calculation. However, association and constraintmust be considered in the above four essential elements of XPS photoelectron signals, resulting in a large extent of coincidence between the fitting envelope and the original curve. Furthermore, the components after peak fitting all possess explicit physical and chemical meanings.