A therapeutic agent containing an isolated recombinant oncolytic poxvirus and NK cells, a kit applying the same, and a method for treating tumors and/or cancer using the same. The therapeutic agent comprises (a) a first pharmaceutical composition containing a recombinant oncolytic poxvirus; and (b) a second pharmaceutical composition containing NK cells. The recombinant oncolytic poxvirus is functionally deficient in a TK gene and a VGF gene, an exogenous IL-21 gene is integrated into the genome of the recombinant oncolytic poxvirus, and the IL-21 gene can be expressed in tumor cells. The recombinant oncolytic poxvirus can produce a synergistic effect on the oncolytic effect of an oncolytic virus and the anti-tumor immunostimulatory effect of the IL-21 gene. The combination of the recombinant oncolytic poxvirus and NK cells produces a further enhanced synergistic killing effect on tumors.