Taking the coaxial double-rotor high-speed helicopter as the research object, a new safety protection and control method based on command constraints is proposed for the safe envelope protection of forward-flight attitude of the helicopter flying at low speed. Firstly, the full-modal nonlinear mathematical model of high-speed helicopter is established. Then, taking pitch-angle protection for the low-speed forward flight as an example, the boundary parameter detection is realized by the dynamic parameter estimation of the restricted parameter and the dynamic trimming algorithm, thus to modify the control input command for safety. Finally, the simulation results show that the control method can protect the flight boundary of high-speed helicopters and improve the safety and reliability of flight.