The invention relates to an ultra-small dual-duct unmanned plane combined navigation system and a dual-mode navigation method. Position, speed and posture information with high precision, high continuity and high reliability is provided for an ultra-small dual-duct unmanned plane, so that the ultra-small dual-duct unmanned plane can acquire information for motion; furthermore, a Bluetooth interface is connected with user mobile Bluetooth equipment to acquire a target location of a user; after the user mobile Bluetooth equipment communicates with a system of the unmanned plane, map information of a start point and a final point is transmitted to a control system of the unmanned plane in real time to finish automatic cruising of a robot. According to a navigation method with dual-mode switching, an error-based adaptive kalman filtering algorithm is used for performing data fusion on all terms of data of an inertia navigation subsystem and a GPS navigation subsystem to obtain optimal navigation information. By control on adaptive factors, continuous operation of the system can be also guaranteed during outdoor and indoor switching of the unmanned plane, so that influence on an output of the navigation system due to failure of single GPS equipment is avoided.