The invention provides a microfluidics chip for detecting mixing with prestored liquid after quantitative flow separation. The microfluidics chip comprises an upper cover sheet, a middle elastic thinfilm and a lower base sheet, wherein the upper cover sheet comprises a pneumatic micro-valve control passage; the lower base sheet comprises a liquid inlet, a liquid outlet, an air inlet of the liquidpassage, a liquid storage tank, a waste liquid tank, a liquid passage, an impedance sensor and an absorbancy testing passage; an open end of the control passage is connected with a pipeline, the pipeline is connected with an electromagnetic valve, the electromagnetic valve is connected with a vacuum system and a pressure system, a PDMS (polydimethylsiloxane) thin film under the passage is controlled to protrude up or press down the liquid passage, and the switching is controlled by software; the air inlet of the liquid passage is connected with an air steel cylinder or an injector; the liquidinlet is connected with a pipeline, and the pipeline is connected with an injection pump; a dissolving agent and thinning liquid are respectively presetored into the two liquid storage tanks. The microfluidics chip has the advantages that the large-ratio sample mixing is realized, the result is accurate and reliable, and a new technology is provided for the quantitative flow separating of the micro-fluid solution.