A method for treating cancer in a subject having a tumor in provided. The method includes administering to the subject a plurality of first dosages, each containing a peptide that comprises a sequence containing SEQ ID NO. 1 and is capable of selectively binding to CXC chemokine receptor 4 (CXCR4); and separately administrating to the subject a plurality of second dosages separately contained from the first dosages, the plurality of second dosages containing an immunotherapeutic agent for treating cancer, wherein the immunotherapeutic agent is an antibody that targets at least one of PD-1, PD-L1, TIM-3, LAG-3, B7-1, B7-H3, NKG2A, KIR, BTLA, VISTA / PD-1H, TIGIT, CD96, OX40, CD28, ICOS, HVEM, 41BB, CD40L, CD137, GITR, CD27, CD30, DNAM-1, CD28H and coreceptors thereof.