Based on the Nonlinear Schr?觟dinger Equation(NLSE), the theoretical model of psssively mode-locked Er3+-doped fluorid fiber laser using a saturable absorber was set up, by which the mechanism of generating mid-infrared ultrashort pulse in the fiber laser was investigated. The numerical simulation focused on the evolution process of the mid-infrared ultrashort pulse in fluorine fiber laser. The impact of the Er3+: fluorine gain fiber length and the saturable absorber unsaturated loss on the generation of mode-locked pulse was analyzed in detail, and the reasonable parameter range was determined. It is found that for a given set of small-signal gain, unsaturated loss, and net intracavity dispersion, the stable mode-locked pulses are achieved by tuning the Er3+: fluorine gain fiber length within a certain range. With increasing the Er3+: fluorine gain fiber length, the output pulse duration decreased gradually, while the increase of the spectrum width (FWHM) and the peak power are observed. Meanwhile, for a given set of Er3+: fluorine gain fiber length, net intracavity dispersion and small-signal gain, the stable mode-locked pulses are also obtained by adjusting the saturable absorber unsaturated loss within a certain range. With the increase of the saturable absorber unsaturated loss, the output pulse duration decreases gradually, however spectral width broadens and then narrows, and peak power increases gradually.