The invention discloses a composite Fe3O4 magnetic nano particle for combining lung cancer cells and a preparation method thereof. Polyimide is modified on the surface of the Fe3O4 magnetic nano particle, and composite protein which has capacity of combining the lung cancer cells is marked on the Fe3O4 magnetic nano particle, wherein the composite protein which has capacity of combining the lung cancer cells consists of staphylococcus A (SPA) protein and one or two of anti-CD44 antibody and anti-CK7 antibody. The invention also discloses a method for enriching and detecting the lung cancer cells in sputum. The method comprises the following steps of: a) collecting sputum, and fully liquefying the sputum; b) adding a composite nano magnetic particle solution and reacting; c) centrifuging reaction liquid to enrich cells in the reaction liquid; and suspending the obtained cells again, tabletting the suspension, dying, sealing and performing microscopic examination by using a thinprep cytology test (TCT) system. Compared with the conventional method, the method has the advantages that the positive rate of the lung cancer cells which is detected in the sputum is improved by more than 30 percent, the detection efficiency is improved obviously and detection cost is reduced to certain extent.