Optical gain characteristics of Ge1?x Snx are simulated systematically. With an injection carrier concentration of 5×1018/cm3 at room temperature, the maximal optical gain of Ge0.922Sn0.078 alloy (with n-type doping con-centration being 5×1018/cm3) reaches 500 cm?1. Moreover, considering free-carrier absorption effect, we find that there is an optimal injection carrier density to achieve a maximal net optical gain. A double heterostructure Ge0.554Si0.289Sn0.157/Ge0.922Sn0.078/Ge0.554Si0.289Sn0.157 short-wave infrared laser diode is designed to achieve a high injection efficiency and low threshold current density. The simulation values of device threshold current density Jth are 6.47 kA/cm2 (temperature: 200 K, and λ =2050 nm), 10.75 kA/cm2 (temperature: 200 K, and λ =2000 nm), and 23.12 kA/cm2 (temperature: 300 K, and λ =2100 nm) respectively. The results indicate the possibility to obtain an Si-based short-wave infrared Ge1?x Snx laser.