Vulnerability analysis is a key step of risk analysis of natural disaster. Based on information diffusion theory and incomplete information, it analyzes the changes of the affected population in urban flood and identifies the function relationship between the hazard (rainstorm) and disaster-bearing body (population), which is called vulnerability curve of population in flood disaster.This curve not only can present some changing extent of urban flood risk, but also could provide an important basis for integrated risk assessment of urban flood.

In the view of systematicness and uncertainty of disaster risk, the paper firstly adopts parameter estimation statistical algorithm, information diffusion model and