Central Liaoning urban agglomeration as the core region of Northeast Old Industrial Base, is on the critical stage of economic transformation process presently. The size structure of the population and economy in the Central Liaoning urban agglomeration is analyzed by fractal method in this paper on the background of revitalizing the Northeast Old Industrial Base, new type industrialization and new type urbanization. There are many arguments on the delimitation of Central Liaoning urban agglomeration area, so this paper tries to make a definition of the confines of the central Liaoning urban agglomeration in two aspects, namely the city gravitational value, and the traffic flow analysis. Compared to the other cities, the gravitational values between Shengyang and Yingkou are smallest, and the socio-economic links of the two cities are weakest. Central Liaoning agglomeration should include Shenyang, Anshan, Benxi, Liaoyang, Fushun and Tieling. Then this paper calculates the fractal dimensions of the population size and economy size. The fractal dimensions of the population and the economy size show that the distribution of population in Central Liaoning agglomeration is uniform, conversely, the distribution of the economy is convergent. The spatial distribution of the economy and population is inconsistent, so the regional economic gap is distinctive. It has a close relationship with the industrial structure and development history of this region. The regional economy is almost capital-intensive industry, it is unable to absorb large number of labor force as labor-intensive industry, so the more developed cities have high economic density but low population density. In order to probe into the fractal mechanism of the population and economy size, this paper uses the grey relational analysis method to analyze the main factors of the fractal dimension. The results indicate that the science expenditure and completion of the total investment in fixed assets are the main effective factors.