A new type of fully passive power generator based on oscillating foil is proposed and numerically tested. The shedding vortexes drive the foil to pitch around the pitching center, and the resulting periodic lift force drives the foil to swing around an arm. Nonlinear spring is utilized to improve the adaptation and energy extraction performance of the power generator. Two-dimensional Navier-Stokes simulation at Re = 1100 are carried out to study the fluid-foil interaction as well as the robustness and performance of energy extraction. Numerical results demonstrate that small pitching amplitudes (0 degrees-90 degrees) lead to higher power coefficient but smaller efficiency and drag force than the large pitching amplitudes (100 degrees-170 degrees). What's more, the range of synchronization is extended from 0.4-1.3 to 0.4-2.5 in terms of speed ratio by the nonlinear spring, indicating that the nonlinear spring can increase the adaptation of the system. Besides, the power coefficient can be improved at some off-design points and reach maximum value of 1.04. In general, the nonlinear spring provides the simplest way to improve the adaptation or robustness of the power generator without increasing the cost and complexity of the system or reducing the performance of the generator. (C) 2017 Elsevier Ltd. All rights reserved.