The invention provides a method for regulating underflow concentration of thickeners and relieving pressure rakes. The method has the advantages that expansive agents with effects of expanding tailings are added to tailing flocculation front ends, the rheological properties of the tailings are changed, the usage of the expansive agents is adjusted, the effects of expanding the tailings to different degrees can be realized, accordingly, the underflow concentration of the thickeners can be adjusted, the tailings can be prevented from being hardened, and the pressure rakes of the thickeners can be relieved; the expansive agents are free of toxicity, low in cost, easy to operate and suitable for popularization and utilization and can come from extensive sources.