The effects of interband transition, free carrier absorption and bandgap narrowing on linewidth enhancement factor (α factor) in semiconductor lasers were comprehensively considered in a simple model. A convenient calculation method of α factor in semiconductor lasers was presented. The formula for α factor was derived at first, the gain of GaAs semiconductor lasers was theoretically analyzed and calculated, and the process of solving the Fermi integral function by taking advantage of the Mupad notebook in MATLAB software was introduced. Further, the effect of interband transition on α factor was calculated based on the peak variation of gain fitting curves. Finally, both the effects of free carrier absorption and bandgap narrowing on α factor were disscused, respectively, and their values were obtained.The results show that interband transition and bandgap narrowing have more obvious effects on α factor in semiconductor lasers (α factor are 22.562 and -6.853, respectively) than the effect of free carrier absorption(α factor is only -0.605).