Abstract: In order to investigate the effects of chitosan on rhizosphere micro-environment of tomato plug seedlings，chitosan was applied to the substrate before sowing，and the rhizosphere bacterial diversities were detected by 454 pyrosequencing. The results showed that pH value of substrate did not change upon chitosan application，a decreased EC value was detected on 40 days after sowing. In the rhizosphere of tomato plug seedlings，the major bacteria genera included Acidovorax（8.92%），Opitutus（5.03%），and Massilia（2.43%）. Application of chitosan could promote the growth of tomato seedlings. After chitosan application，the rhizosphere bacterial comunities changed markedly，and the bacterial richness and diversity decreased significantly. The dominant genera included Pelomonas（13.52%），Cellvibrio（8.21%），Rhizobium（2.91%），and Nitrosospira（1.04%）. Most of these bacteria were potential plant growth-promoting bacteria. Taken together，chitosan could selectively promote the proliferation of benifical bacteria，and thereby improve the bacterial community in the rhizosphere substrate of tomato plug seedlings.

Key words: tomato, substrate, chitosan, rhizosphere, bacterial diversity