Abstract: The development of microspherical cavity theory is introduced, and the microspherical cavity is analyzed with light quantum mechanics. The 1533nm fluorescence spectrum of the Er3+ doped phosphate glass microsphere (about 15μm in diameter) was measured under laser excitation of 488nm. The phenomenon of the morphology-dependent resonances of the fluorescence spectrum in the glass microsphere and its formation mechanism were observed and analyzed. The fluorescence spectra of the Er3+ doped phosphate glass microsphere were compared with those of corresponding bulk glass. The single-mode oscillation amplitude generated by the Er3+ doped phosphate glass microsphere was obtained in the experiment.