During 21th century , the objective of development strategy of Shanghai is to be an international metropo2 lis , so the scale of population must be adaptable to it . Determining the rational scale of population is a complex problem , it concerns many aspects , such as the level of economy , ecological environments , etc. And each aspect includes many factors , which are interdependent and interactive. This paper studies the reasonable population scale in Shanghai based on the PS aided decision system (PSDAS) . Qualitative and quantitative analysis are done about traffic , environment and other factors. This paper also analyzes , calculates , displays and outputs the result on the computer. This paper gets the reasonable population scale in 2050 in Shanghai and relevant conditions. The sensi2 tivity analysis was used to find out the crucial factors that influence the reasonable population scale in Shanghai greatly. By comparison and analysis of many schemes , we got a diagram of curves of the scale of population , which provides a scientific basis for policymakers to make a strategic decision