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A formula for the threshold energy of a four-level-system laser in Tm3+ doped fluoride glass is derived. The formula contains the parameters of the interaction between the ions and the up-conversion rate of the pump-light. It is discussed that the Tb3+ ions and the up-conversion pump-light have an influence on the laser performance of Tm3+ ions around 1.47μm wavelength. We draw the following two conclusions. (1) The laser threshold descends obviously due to the doping of Tb3+ ions, but the descending gradually flattens out with the increase of the content of Tb3+ ions. Its suitable content is (1-2)× 1020cm-3. (2) When pump-lights both from the up-conversion and from the ground state excite simultaneously the single-doped Tm3+ ions, the threshold energy can reduce and the output energy can increase. In this paper, the energy conversion efficiency from the up-conversion pump-light to the output energy of laser is estimated.