A semiconductor laser that generates a chaotic signal with optical feedback can be treated as a chaotic transmitter. Based on single mode rate equations of semiconductor laser with optical feedback and external injection, the numerical simulation shows that the bandwidth of the transmitter is expanded by external optical injection. When the injection index kinj is 0.39, the bandwidth is increased from the value of 2.7 GHz without optical injection to 14.5GHz. The results also reveal that, under the same injection strength, the enhancement of bandwidth depends evidently on the frequency detuning between the external injection laser diode and the chaotic carrier transmitter. The maximum bandwidth of the chaotic transmitter can be obtained when the frequency detuning is in the range from 2 to 4GHz. The bandwidth of the chaotic transmitter can also be enhanced by increasing the bias current.