Based on Population Development Equation which is amended by the "equal-dimension gray dynamic model filling vacancies in the proper order", the paper establishes a metropolis population forecast model and forecasts the population trend of Chengdu city in the period of 2005 to 2030. The results of the forecast demonstrate that in future the aged population structure and the trend of population aging up would never be changed despite amounts of immigrants increasing to the city if the current low fertility rate were kept for a long time. On the basis of this conclusion, authors offer several corresponding suggestions.