The present paper proposed for the first time the flocculation-filtration method for separation of interfering substances in milk, such as fat. In this method only two steps were carried out. Firstly, aluminum chloride (PAC, Al2(OH)nCl6-n) is used to flocculate the milk; Secondly, water filter was used to filter the mixture. Then the clear filtrate could be used for the detection of melamine. The whole preprocessing would not take more than one minute. The pretreatment process was optimized. Experiments show that the adding proportion of PAC should be about 2%～3% for best filtration efficiency, and that it would have the best flocculation effect when the mixture was mildly alkaline. High performance liquid chromatography experiments show that the melamine recovery of this method is more than 90%. Samples pretreated by the flocculation-filtration method were clearer and the baselines of spectral curve obtained by sensitizing Raman method were more smooth which means better purification compared to those samples pretreated by centrifugal pretreatment method. The pretreatment method proposed can be used in HPLC and Raman spectroscopy methods for rapid detecting melamine in liquid milk. This method shows better separation effect, simpler operation, and lower time and money cost than those pretreatment processes in the existing standard melamine detection method for milk. By use of this pretreatment method, the melamine rapid detection efficiency would be greatly improved.