The absorption spectrum and fluorescence spectrum of Tm:YAG crystals have been measured. The oscillator strength of absorption transitions have been calculated. According to the Judd Ofelt theory, the spectral strength parameters Ω 2= 0.5×10 -20 cm 2, Ω 4 =1.0×10 -20 cm 2, Ω 6 =0.4×10 -20 cm 2 of Tm 3+ have been fitted. Then the spectral parameters of Tm 3+ including spontaneous emission oscillator strength, transition probabilities, radative lifetime, fluorescent ratio and integral emission cross-section have been calculated. Pumped with 694.3 nm ruby laser, the laser pulse of 40 mJ at 2.01 μm was obtained.