The nonlinear refractive index n2 of a PbO, TiO2, SiO2, and K2O quaternary glass system measured by the laser beam longitudinal scanning (Z‐SCAN) method was found to be 7.72×10-19 m2/W and it was strongly correlated with the presence of more polarizable ions rather than the nonbridging oxygen content. The structural units of the present glass system has been studied by infrared (IR), Raman spectra, and x‐ray photoelectron spectroscopy (XPS) techniques. The results indicate that the IR absorption band at 1020 cm-1 and the Raman peak at 1100 cm-1 attributed to the stretching vibration of the Si—O—Si bond shift to a lower wave number with an increase of the relative content of K2O. The XPS O1s spectra are decomposed into two main components, which reveal that oxygen atoms of the glass network occur in both bridging and nonbridging oxygen with binding energies of 531.77±0.30 eV and 530.17±0.40 eV, respectively. Si4+ ions are in the form of SiO4 to form the glass network, Ti4+ ions are dual 4‐ and 6‐coordinated with oxygen ions, K+ ions act as network modifiers, and Pb2+ as network modifiers at a low content of PbO, but are network formers at high content. A structural model has been proposed.