The invention discloses an unmanned aerial vehicle autonomous navigation system and a navigation method. The system comprises a parameter navigation module, a radar navigation module, a central processing module, an information storage module and a heading control module, wherein the parameter navigation module comprises a GPS receiver, an angular rate gyroscope, an IMU module, a rudder deflection potentiometer, an image acquisition unit and a timer. The navigation method comprises the following steps: when GPS signals are instable or are lost transiently, state parameters outputted by the parameter navigation module are processed in the central processing module to obtain flight parameters; when the GPS signals are lost for a long time and the parameter navigation module can not be used for navigation, the radar navigation module is used for navigation; and when the GPS signals are lost for a long time, the parameter navigation module fails and radar signals can not be received, the central processing module is used for path planning. The system and the method provided by the invention can provide accurate navigation for the unmanned aerial vehicle in a condition without GPS signals.