The intensity of upconversion emissions of rareearth ions is determined by the maximum phonon energy of glass host. In this paper the intensities of upconversion emissions in Yb3+/Er3+codoped germanatetellurite glas ses upon the excitation of 980nm laser diode were extremely enhanced by replacing PbO wit h Bi2O3 and increasing the size of alkalimetal ions. Rama n spectra show tha t the maximum phonon energies of the glass hosts investigated keep unchanged, wh ile the maximum phonon density decreases on substituting Bi2O3 for PbO and i ncreasing the size of alkalimetal ions. Based on the nonradiative decay rate, it is found that the maximum phonon density is the main reason for the enhanceme nt of upconversion emissions in Yb3+/Er3+codoped germana tetellurite glasses.