The present invention relates to the application of the metabolite Tagalsin C of marine life and its homologs Tagalsins A, B, D, E, F and G in preparing medicine for treating tumor. The series of compounds has high membrane permeability and broad spectrum antitumor activity of killing human promyelocytic acute leukemia cell HL-60, human myeloma cell IM9, human acute T-cell leukemia cell Jurkat, human macrophage lymphoma cell U937, human lung cancer cell A549, etc, and no obvious injury on health human nephridium cell 293aa human liver diploid cell L02. The series of compounds may be used in preparing medicine for treating tumor, medicine with high clinical Bcl-2 expression and conventional chemotherapy resistance, and medicine for treating cancer recurrence.