The invention provides a straw and biogas slurry synchronous returning method and a machine, and belongs to the farmland preparation technology and machines. The method includes the steps that crop straw and stubbles in fields are smashed and scattered on the fields, biogas slurry generated after anaerobic fermentation of biomass is sprayed on the smashed straw and the stubbles, soil is turned over and tilled by means of a moldboard plow, the smashed straw, the smashed stubbles and the biogas slurry are buried in and covers the soil together, and finally the surfaces of the fields are compacted and leveled. The machine comprises a machine frame assembly, a three-point suspension assembly, a straw and stubble smashing cutter shaft assembly, a walking wheel assembly, a biogas slurry spraying assembly, a moldboard plow assembly and a compacting assembly, wherein the three-point suspension assembly, the straw and stubble smashing cutter shaft assembly, the walking wheel assembly, the biogas slurry spraying assembly, the moldboard plow assembly and the compacting assembly are arranged on the machine frame assembly. The method is simple in process, the straw returning effect is good, quality is high, soil restoration is facilitated, work cost is low, the machine is reasonable in structure, the use requirement for straw and biogas slurry synchronous returning is met, work efficiency is high, and usage is reliable.