Steam flooding can greatly enhance the heavy oil recovery. However, for single steam flooding, it is possible to decrease the steam sweep efficiency resulting from poor heavy oil development efficiency due to steam channeling and heterogeneity. According to the characteristics of the heavy oil reservoirs in Block Q40, designed are two different injection modes of nitrogen foam-assisted steam flooding after single steam flooding. Analyzed are the displacement pressure, instantaneous (cumulative) oil and gas ratio, water cut and oil displacement efficiency and obtained is the EOR oil displacement mechanism of nitrogen foam-assisted steam flooding. Meantime, developed is a double long core model to conduct simulation experiment on nitrogen foam-assisted steam flooding in the conditions of 4 different streamline ratios through changing double core length, analyzed is the effect of different streamline ratios on the development index and obtained is the polynomial function of streamline ratio and oil displacement efficiency according to the experimental data. Experiment results show that nitrogen foam-assisted steam flooding can further enhance the ultimate recovery after the single steam flooding, recovery degrees of different streamlines in the same flow field will decrease with streamline length increase and oil producing capacity of the injection- production main stream line accounts for more than 50% of the total oil output.