Early diagnosis plays a decisive role in the outcome of colorectal cancer (CRC) therapy. The ex vivo culture of fresh CRC tissues and paired normal colorectal tissues provides a feasible way to explore potential serum biomarkers for CRC early detection under near-physiological conditions. In the present work, we applied a lectin affinity based approach to enrich and increase the detection number of secreted proteins in the conditioned media of cultured tissues. The captured proteins were then analyzed by the proteomic strategy of one-dimensional gel electrophoresis coupled to liquid chromatography-tandem mass spectrometry. By quantification with label-free spectral counting, we found 123 differentially expressed secreted proteins (DESPs) with 68 DESPs up-regulated in CRC tissues. EFEMP2, one of the top 10 up-regulated DESPs, was further validated by immunohistochemistry at tissue level and enzyme-linked immunosorbent assay at serum level. We found the expression level of EFEMP2 was dramatically increased in CRC patients, even at the early stage. Moreover, the diagnostic accuracy of EFEMP2 was superior to the established CRC biomarker carcinoembryonic antigen evidenced by the area under the receiver operating characteristic curve for the two biomarkers were 0.923 and 0.728, respectively. These results indicated EFEMP2 is a promising serum biomarker for CRC early detection.