Based on the data about the rural people in the South Jiangsu, this paper studies the impact of decrease of family size on the economic security capacity of families by using the method of model simulation.If income accumulation does exist, the decrease of family size will not weaken the security capacity of families, but strengthen it.The difference of security capacity due to different family size increases with the increase of consumption elasticity, while it decreases with the increase of risk occurring.And it also fluctuates at different stage of family life period.Family security capacity varies due to the differences in family size, consumption and accumulation pattern, and being at different stage of family life period.