The changes of soil organicmatter，cellulase activity and bacterial diversity after deep ploughing of maize straw werepreliminarily studied by means of routine analysis and variable gradient gel electrophoresis(DGGE).The results showed that withthe advance of maize growth period，the content of soil organic matter increased firstly and then decreased.The content of soilorganic matter was higher at jointing stage and big trumpetstage.Soil organic matter content in 0～ 20 cm soil layer with deepploughing of maize straw for oneyear(SF -Ⅰ)and twoyears(SF -Ⅱ)had no significant difference except sowingstage，but hadsignificant difference withCK.Soil organic matter contents of SF -Ⅰ，SF-Ⅱand CK in 20～ 40 cm soil layer were not significantlydifferent except at sowingstage.The content of soil organic matter in maize mature stage was SF -Ⅱ> SF-Ⅰ>CK，and the contentof soil organic matter decreased gradually with the deeper soillayer.The soil cellulase activity increased significantly after strawreturning tofield，showing as SF -Ⅱ>SF-Ⅰ>CK；Soil cellulase activity increased at jointing stage and big bellstage，and thedifference with CK reached a significantlevel.The activity of cellulase decreased gradually with the deeper soillayer.In the0～ 20 cm soillayer，the soil cellulase activity from sowing to trumpet stage had significant difference between straw returning andCK，but had no significant difference in the otherperiods.In 20～ 40 cm soillayer，only sowing stage had significantdifference. 16S rDNA-PCR-DGGE analysis showed that soil bacterial community changed greatly from jointing stage to filling stage after strawreturning，and the change was the biggest at jointing stage.SF -Ⅱ，SF-Ⅰ and CK had different soilbacteria，and bacterial diversity index was SF-Ⅱ>SF-Ⅰ>CK.Deep ploughing of straw increased soil bacterialdiversity，especially cellulose decomposingbacteria and organic pollutant degradingbacteria.In the semi -arid coldarea，straw returning to field for two years is better than straw returning for one year.