The chaotic communications systems with optical feedback lasers in strong injection and open-loop configuration are studied in this paper. The quantity relationship between gain and injection factor is obtained by numerical calculation. The demodulators performances under the conditions of whether the gain is used to adjust the output of the receiver are compared. The result indicates that the recovered message is more close to the original message by using the adjusted subtraction demodulation method. It is proved that message rate has no effect on gain by testing sine signals with different rates. Further，the gain is shown to be independent of message type and encryption mechanism by testing digital signals at 0.2?Gbit/s rate.