In this article KMR theory is used to calculate the free-electron laser at 10.6?m and the effects of current density, laser intensity and energy distribution of initial electron beam are discussed. The results of computer simulation is given to compare several groups of different experimental parameters. It can be used to guide tho design on free-electron lasers, to determine the requirements on electron beam quality and Wiggler parameter. The gain of the radiation is expected to be 1~3%.