Due to the advantages of low ripples in input and output current, and bidirectional energy flowing for the Cuk converter, it shows good application prospects both in new energy generation and DC micro-grid. Based on the analysis of the converter's working principle, the turn-on and turn-off switching flow graphs are established respectively. Then, the switching branch models realized with multipliers to derive the switching flow graph in the whole switching period. Finally, the small signal model of the converter is obtained by imposing disturbance on the switching branches, and the transfer functions of the Cuk converter are calculated with Mason's gain formula. The correctness of the proposed model is proved by simulation results of the AC analysis on the converter, which is implemented with Power Simulation(PSIM) software. The modeling method in this paper has a high reference value for investigations on high order DC/DC converters' modeling.