The invention provides a farmland treatment method for straw decomposition and prevention and control of crucifer crop clubroot. The method comprises the following steps that when rapes are mature and harvested mechanically, crushed straws directly enter into a farmland, a Bacillus atrophaeus BA-7 bacterial agent is directly applied to the farmland, and the application amount of the Bacillus atrophaeus BA-7 bacterial agent is 500 to 2000 billion bacteria per mu (a unit of area equal to 0.0667 hectares); the Bacillus atrophaeus is classified and named as Bacillus atrophaeus BA-7 and collected in China Center For Type Culture Collection (CCTCC) on December 21th, 2015, and the collection number is CCTCC NO: M 2015765; the Bacillus atrophaeus BA-7 bacterial agent is the bacterial agent prepared after the Bacillus atrophaeus BA-7 is fermented. According to the invention, pile fermentation is not required to be performed on the straws, so that the high-efficiency decomposition of the straws can be facilitated and the crucifer crop (especially the rapes) clubroot can be prevented and controlled. The farmland treatment method disclosed by the invention has the advantages of simple using method, low treatment cost, no environmental pollution and high-efficiency prevention and control of the clubroot to improve the crop yield.