Using the annual unbalanced panel data of SMEs in Jiangsu province from 2007 to 2013 in BvD Asia-Pacific Enterprise analysis database, this paper analyzes the relationship between the competition degree in banking industry and the credit availability of SMEs，and explores the important regulatory role played by market size structure. The results show that market size structure is an important factor affecting the relationship between the degree of banking competition and the availability of credit for SMEs. When large banks dominate the credit market， the improvement of competition is conducive to the financing of SMEs，and the credit availability of SMEs increases monotonously with the competition of banking industry. When small banks dominate the credit market，there is an inverted U-shaped relationship between the credit availability of SMEs and the degree of market competition. The existing research conclusions on the relationship between banking competition and SMEs financing are quite controversial，and the neglect of the key factor of market size structure may be an important reason for the great divergence in existing research.