Helicopters have been playing an increasingly important role in military and civilian activities. The review of helicopter noise reduction is needed to be updated， because new technologies are constantly emerging. Firstly， the mechanism and propagation properties of the rotor （tail rotor） noise are described， and the co-axial rigid rotor high-speed helicopter and the tilting rotor are introduced. There are complicated aerodynamic or acoustic interaction in two new configuration helicopters. Secondly， the progress of helicopter flight noise measurement， as the verification approach for low noise design technologies， is demonstrated. Then， the design technologies for silent helicopters are summarized. The passive noise reduction strategies have been intensively implemented， and reduced helicopter radiated noise level greatly. The strategies of variable rotor speed and low noise trajectory are gradually maturing. However， the active control technologies are in the laboratory stage， and need further research on driving devices and control parameters et al. The final part summarizes the state of art for low noise control strategies in helicopters， and outlooks the development trend.