The generalized synchronization (GS) of Duffing system is investigated, then a digital secure communication scheme based on the generalized chaotic synchronization is proposed. The binary messages to be transmitted are firstly modulated by FSK modulation, then the FSK signals drive non-autonomous chaotic system to transmit chaotic signals. In the receiver, the binary messages can be recovered when the chaotic systems achieve generalized synchronization. Simulation results show effectiveness of the scheme.