For the complex fault block reservoir with edge water，it is impossible for foam displacement to prevent edge-water coning because of such a long distance，while the foam huff-n-puff just can solve this problem. In order to confine the mechanism of foam suppression for edge water in huff-n-puff wells，the effect of nitrogen，surfactant and polymer should be known clearly. The foam system’s stability，adsorption properties and sealing performance was evaluated，then a related experimental model was developed to evaluate the effectiveness of nitrogen foam preventing edge water coning in huff-n-puff wells by comparing with that of single nitrogen system and surfactant-polymer system. The experimental results showed that the selected nitrogen foam system had good adaptability with the formation. The interfacial tension between the system and the simulated oil was 1.097 mN/m and the foam index was 14750 mL·min at the temperature of 85℃ and salinity 4176 of mg/L. The resistance factor was 26—103 correspond to the formation permeability of 500×10^-3—1500×10^-3 μm². The main mechanism of nitrogen foam in puff and huff wells was as follows，the injection of nitrogen could keep the reservoir’s pressure in a high level；the injection of surfactant and polymer could control the gas flow of nitrogen and reduce oil and water interfacial tension to a certain extent. When the foam is shaped by mixture of gas and liquid，the before-mentioned shortages of gas，surfactant and polymer would be canceled because of their co-reaction，which made nitrogen foam have its own superiority in preventing edge water coning in puff and huff wells.