Several structures such as footbridges, stadiums and floors are subjected to human walking load. Due to aesthetic demand and new construction technology, these structures are getting slender and as a result prone to large amplitude vibration. In order to study these cases, an appropriate force model of pedestrian is required. For a pedestrian, the walking motion is self-sustained and periodic in nature. Moreover, it produce the required energy itself to maintain its motion. Therefore, the pedestrian can modelled as a self- sustained oscillator, which produced three phenomena: (i) self-sustained walking motion (ii) periodic lateral force signal and (iii) stable limit cycle. In the present study the proposed model has been derived from experimental treadmill data of walking pedestrians. Further, the proposed model is a modified form of van der pol Rayleigh oscillator with additional nonlinear terms. Computed results from this study have a close agreement with experimental data.