Common-signal-driven synchronization in two semiconductor lasers has been proven to achieve secure key distribution, but a high-level correlation between driving and response signal reduces the security. In this paper,we demonstrate a secure key distribution scheme based on a transformation module to enhance the protection efficiency of chaos synchronization-based secure key distribution. The chaotic signal is subjected to a delayed interference by the Mach-Zehnder interferometers (MZI) to eliminate the residual correlation in intensity. Subsequently, the final keys were generated by sampling and quantizing the chaotic waveforms from MZI utilizing the dual-threshold method. A proof experiment for secure key distribution at the rate of 0. 23 Gb/ s through 100 km fiber transmission was implemented.The experimental results prove that the transformation module introduced in the chaos synchronization-based key distribution scheme can effectively prevent an eavesdropper extracting keys directly by intercepting the output waveforms from the intensity variation of the driving source and realize the secure key distribution for legitimate users.