The structure of Tm3+-doped langasite(La3Ga5SiO14，abbreviated as LGS) crystal has been measured by XRD. The crystal lattice parameters were calculated with DICVOL91 program. The absorption spectra and emission spectra (excitated under 470nm) of Tm：LGS single crystal was measured at room temperature. And the optical parameters, such as transition-line intensity, oscillator strength and absorption section were calculated by using Judd-Ofelt theory. The three intensity parameters Ωt(t=2，4，6) were fitted to be 2.694×10-20cm2，1.842×10-20cm2，0.030×10-20cm2，respectively. The lifetime, branching ratio and integrated stimulated-emission cross section were obtained and the results were discussed.