Based on a primal dual path following interior point method， a steady state security constrained optimal power flow approach is presented. How to incorporate multi-contingency steady state security constraints into classical optimal power flow model and which variables should be considered as control variables are analyzed. According to the perturbed KKT conditions of the primal problem， a primal dual path following interior point method is directly applied to solve this large-scale nonlinear programming（NLP） problem. Bordered-blocked-diagonal-form of the reduced KKT system is revealed， in which each diagonal block is constructed by 4×4 small block elements and has the similar frame of the nodal admittance matrix. The dimension of the reduced KKT system only depends on power flow equations. Complexity analysis suggests that each iteration's computation burden is about 8 times than that of Newton power flow iteration for base case and binding contingencies at the same time.