The degradation of the cathodoluminescence (CL) brightness of the phosphor screen during the panel sealing process with frit glass as a sealing material was investigated. A simplified outgassing process, which could offer the same environment as the standard panel sealing process to the phosphor screen, was designed to investigate the searched effects with ease and accuracy. Commercially available red, green, and blue phosphors, i.e., Y2O2S:Eu, ZnS:Cu,Al, and ZnS:Ag,Cl phosphors were used in this study, and the characteristics of CL degradation varied with the phosphor. It was found that the CL brightness of the phosphor screen decreased after experiencing frit outgassing during the standard panel sealing process and the simplified outgassing process. From the surface analyses on the phosphor screen, it was found that the CL brightness of the phosphor screen decreased because of the surface contamination due to PbO and CO2 released from the frit.