To describe the behavior characterization of a MEMS(Micro-electro-mechanical System) resonator with a large amplitude accurately, a lumped parameter model of the nonlinear stiffness MEMS resonator was established and an innovative method to measure and characterize the nonlinear coefficient of the resonator was presented. A closed loop control circuit was designed and manufactured based on a Phase Locked Loop (PLL) and an Automatic Gain Controller (AGC) and the quantity relationship between the vibration amplitude, vibration frequency, and the nonlinearity coefficient was given. Finally, according to the the given relationship, the natural resonant frequency and the stiffness nonlinear coefficient were measured by curve fitting in both of decaying mode and stable mode. The results indicate that the repeatabilities of the two measured results are 18.6×10-6 and 1.5%, respectively. With the measured amplitude self-decaying curve, the residuals were measured by an ideal second-order system resonator model and a stiffness nonlinear resonator model, and the results show that residuals from the latter is less 9.5% than that of the former. The results demonstrate that the stiffness nonlinear resonator model is more close to the actual situation and verify the validity of the measured nonlinearity characteristics by the proposed method. Moreover, the temperature characteristics of nonlinear coefficient and natural resonating frequency were also investigated. The temperature coefficient of the natural resonant frequency is -0.487 Hz/℃, and the goodness of linear fitting is up to 99.964%.