This paper presents an adaptive dual closed loop speed control strategy for natural gas engine used in a range extender to improve the robustness against parameter uncertainties, unmodeled dynamics and unknown disturbance. The outer loop of the strategy is the speed control, the output of which is the target intake manifold pressure. The inner loop is the adaptive control of intake manifold pressure, and the output is the angle of the throttle valve. The strategy is realized in simple structure without the knowledge of the specific values of the engine parameters. Theoretical analysis indicates that the controller can meet the demands of the wide range speed regulation of the range extender engine in the presence of model errors and unknown disturbance. Finally, the effectiveness and practicability of the controller are verified in both simulation in the environment of Matlab/Simulink and experiment in a range extender.