For diesel vibration isolation nonlinear system, curve fitting method based on Hammerstein model is proposed, which can discuss how nonlinearity affects features of vibration system and can obtain linear structural dynamical features by implementing mode test. Generalized Frequency Response Functions (GFRFs) is established based on Hammerstein model, and (the 1st generalized) FRF of the system is extracted when considering fundamental frequency harmonic GFRFs’ impact on fundamental frequency response. (The 1st generalized) FRF is estimated by employing curve fitting method in frequency-domain, and linear structural dynamical feature is identified. The experimental result of mode parameters identification showed that the proposed method was valid and feasible to estimate linear mode parameters of diesel vibration isolation nonlinear system. When carrying out curve fitting based on Hammerstein model, one can eliminate nonlinearity effects on features of vibration system, and can obtain linear structural dynamical feature of nonlinear system.