Firstly, the paper establishes a brand-extension size model and adopts the marginal analysis to analyze the optimal solution of it. Secondly, research the brand-extension condition of G brand by questionnaire survey. There are 240 questionnaires handed out, with 211 withdrawing and 187 valid, by doing this to test the influencing degree of each factor in brand-extension size model; At the same time, use analytic hierarchy process to analyze the size of brand extension and principal component analysis to find out the main factors that influence brand extension of G brand. Finally, the paper makes an empirical analysis on brand extension size model based on ecological characteristics, the results show that the order of the total information reflected by each principal component which obtained from the principal component analysis is somewhat different from that of analytic hierarchy process; With the continuous improvement of living standards, the consumers pay more attentions to the convenience of shopping and comfort of shopping environment day by day. Therefore, the brand enterprises must strengthen environmental construction of brand group and satisfy the growing material and cultural demand of consumers. Success rate of brand extension to a great extent has a close relationship with the products and product channel, compared to foreign brand, consumers are relatively weak in brand communication and association of the products.