The utility model relates to a mixed gas separating system in MTP (Methanol to Propylene) reaction. Chilled water in a kettle of a pre-quench tower and chilled water in a kettle of a quench tower are respectively processed; gas hydrocarbon received from a compression section II enters a compression section III through water washing and alkali cleaning, and the gas hydrocarbon obtained from a compression section III and liquid hydrocarbon not containing heavy hydrocarbon above C6 are delivered to a depropanizator through gas hydrocarbon drying and liquid hydrocarbon flocculating and drying; a flocculated liquid stripping column is arranged among the compression sections for separating the flocculated liquid among the compression sections, light hydrocarbon returns to the outlet of the compression section II, and heavy hydrocarbon is delivered to a debutanizer for refinement; the depropanizator is arranged between the compression section III and the compression section IV and adopts conventional precise distillation; and a differential condensation device is arranged at the top of a demethanizer for reclaiming self cold energy and reducing ethylene loss rate. The mixed gas separating system can efficiently optimize technological process, lower the energy consumption of the system, reduce the corrosion of pipelines and reduce the cost of investment.