Since radar signals are modulated by rotor blades shadowing and helicopter vibration, the helicopter-borne synthetic aperture radar imaging is difficult. Modulation reasons are analyzed and investigated based on time-frequency method. The modulation phase errors are extracted from the dechirped data and then compensated to remove modulation. Subsequently the phase gradient autofocus is performed to achieve high quality SAR image. This method has great reference value to the helicopter-borne SAR imaging.