The powder and single crystal of Na5Er (WO4)4 have been prepared and their lattice parameters, absorption, excitation and fluorescence spectra have been measured. Meanwhile, the oscillator strength has been calculated from the absorption spectrum. According to Judd-Ofelt theory the spectral strength parameters Ω2=1. 74×10-20 cm2, Ω4 = 1. 46 × 10-20 cm2, Ω6 = 0. 51× 10-20 cm2 of Er3+ have been fitted. Then the spectral parameters of Er3+ including spontaneous emission electric- dipole and magnetic-dipole oscillator strengths, transition probabilities, radiative lifetime, fluorescent branch ratio and integral emission oross-section have been calculated. Finally, the probalility of emitting laser radiation of Er3+ is discussed.