The invention discloses a combined application of a cystatin SN and a carcino-embryonic antigen (CEA), and particularly discloses the application of the cystatin SN and the CEA in preparation of a marker for detection of gastric cancer or gastrointestinal stromal tumor. The invention also discloses a capture agent of the gastric cancer or gastrointestinal stromal tumor marker and a kit containing the capture agent. The prepared kit has the advantages of good specificity, high sensitivity and the like, can be used for early diagnosis of gastric cancer or gastrointestinal stromal tumor, therapeutic effect evaluation in a therapeutic process and monitoring of metastasis and recurrence after therapy, and has test results sooner than clinical symptoms.