The optimal operating condition of gain switched semiconductor laser with different modulation rate is studied in this paper. The theoretical results point that the optimal operating condition is quite different for the variation of modulation rate. In the condition of low modulation rate, small bias current should be applied to avoid the tailing of output pulse. In the condition of high modulation rate, large bias current should be applied in order to improve the extinction ratio of the pulse. The theoretical results agree well with the experimental results.