There is a high tolerance on parameters mismatch between transmitter and receiver in the chaos communication system based on injection locking synchronization，so to a certain extent，the information security has been reduced. We propose the synchronous modulation of feedback strength to improve the system security. The change of signal extraction quality after modulation is studied numerically，and the effects of modulation rate detuning and modulation delay time on signal extraction are analyzed too. The research results indicate that the synchronization of the original system is destroyed and the bit error rate increases by two orders of magnitude when the transmitter feedback strength is modulated. The information can not be recovered unless the receiver uses the same modulation. Furthermore，the results also show that high quality information could be obtained when rate detuning and delay time are in the range of ±5 MHz and ±0.1 times the modulation cycles, respectively. All of these prove that modulating the feedback strength can enhance the security of chaos communication efficiently.