The invention relates to a method for improving the extraction yield of black liquid. A production line for manufacturing wheatgrass pulp with the daily output of 150 tons is inserted with a plurality of intermittent-type filter presses for carrying out parallel squeezing to extract the black liquid of over 70 percent; and the black liquid of over 70 percent enters a vacuum rotary drum to extractthe black liquid by counterflow. The method is characterized in that a programmable controller (PLC) is used to control an electromotive (electric) valve in a feed port or a discharge port of the filter press and a technological process; the parallel filter presses are tightly linked with the product line; and the flow rate is matched with the output. According to the method, four or five sections of the production line for extracting the black liquid wheatgrass pulp by the vacuum rotary drum are reconstructed; and the extraction yield of the black liquid reaches over 95 percent, thereby reaching or approaching the domestic level of the clean production of wood pulping.