Complex resistivity method is based on the characteristics of spectral parameters of rocks and ores, and the multiple spectral parameters provide more information for evaluating anomalous bodies. However, the inverse problem of SIP has not been resolved well, so that it has restricted the application and development of SIP. Therefore, we derive the formula for the 2.5D SIP responses, which starting from maxwell’s equations combined with the Cole Cole model, then we complete the forward modeling with finite element method. The forward method can simulate SIP response contains the electromagnetic effects with topography. On this basis, we propose a combined inversion method by using multi arrangement of apparent resistivity and apparent phase of SIP date, and the smooth model constraint of Occam method is used to improve the stability of inversion. The algorithm can perform large scale inversion, to deal with largely underdetermined inversion problem. The numerical test show that this method can be used to inverse all SIP parameters of geological model with synthetic data (noiseless data or noise data), which indicates the inversion algorithm is feasible.