There is a significant narrowing of the lateral radiation spectrum of the substrate of organic light-emitting diodes as compared to the forward radiation spectrum. Studying the factors that affect the lateral radiation spectrum narrowing of the device and further reducing the spectral linewidth can provide a foundation for the study of the electrically pumped organic light-emitting diode laser radiation. We study the full width at half maximum, peak wavelength, and polarization characteristics of lateral radiation spectrum in organic light-emitting diode substrate, with the thickness changes of hole transport layer NPB. The lateral radiation spectra of organic light-emitting diode with Ag film evaporated on both sides of the substrate edge are compared with those of organic light-emitting diode without Ag film. The full width at half maximum of the lateral radiation spectrum with Ag film is narrower. When the NPB thickness is 130 nm, the full width at half maximum of the lateral radiation spectrum in the device substrate reaches its narrowest, which is 14 nm. This shows that the optical resonator will affect the light propagating laterally in an organic light-emitting diode substrate when mirrors are provided on both edge sides of the substrate. The results indicate new approaches to narrowing the radiation spectrum and amplifying the light of organic light-emitting diodes.