Taking Tianjin power grid as an example, this paper analyzes the characteristics of power demand growth in different stages of economic development in recent 20 years, uses Granger causality test to verify the causality between economy and power demand, and sums up the main factors that affect the growth of power demand. This paper uses ARMIR model to forecast the economic gross of Tianjin, and forecasts the power demand by using the vector autoregressive model based on the economic forecast. The rationality of power demand growth is verified by the H-p filter theory from the angle of economic cycle and power cycle.