The terahertz(1012Hz)spectra, Raman spectra and infrared-visible-UV spectra are analyzed for PbB4O7 crystal sample. A resonance peak appears in the curve of the dielectric function ε(ν) in the frequency range of 0.25—2.5 THz. The analysis supports that soft optical phonons exist in PbB4O7 crystal from four aspects: 1) the change curve ε(ν) of the dielectric function satisfies the Lyddane-Sachs-Teller relation; 2) a strong Raman scattering peak appears in the vicinity of resonance frequency (3.10 THz); 3) absorption coefficient curve α(ν) meets the characteristics of polaritons; 4) the dispersion relation curve ν(k) of the photon transmitting from the crystal reveals a frequency gap. The soft optical phonon exists in PbB4O7 crystal, which means that the frequency curve of the photon transmitting from the crystal will suffer splitting when the condition of producing polariton is satisfied. And the dispersion curve splits a higher branch and a lower one. This may help us find a new method of changing the frequency of the photon.