In this paper, a novel four-dimensional mixed-order chaotic system is proposed and numerical simulations are first carried out through bifurcation diagrams and Lyapunov exponent spectrum. Based on the fractional-order approximate transfer function, the mixed-order chaotic circuit is designed, and the results of the circuit implementation are given. The existence and physical realizability of the new four-dimensional mixed-order chaotic system are proved. Finally, the active control method is used for synchronization of two same mixed-order systems. Numerical simulation results show that the active control method is easy to implement and reliable for synchronizing the two nonlinear fractional-order chaotic systems.