The invention relates to a method for cyclicaly separating drilling fluids during an underbalance drilling operation on sulfur-bearing strata, belonging to the technical field of processing drilling fluids used for sulfur-bearing oil gas wells. The separation method includes the steps of pressure control, oil, solid and gas separation, natural gas separation, sulfureted hydrogen removal, solid control and slurry proportion; and the equipment comprises a drilling fluid seal and separation system and a solid control system. The invention provides the method for cyclicaly separating the underbalance drilling fluids of the sulfur-bearing strata, which can effectively control the sulfureted hydrogen in the drilling fluids, and seal and separate stratum products; and the method solves the problems that the open treatment of the prior art generates threat to people and harm to the environment, and the method has the characteristics of complete seal treatment, cyclic utilization and on-line monitoring and accurate measurement of the stratum products with the information of the stratum conditions.