Disclosed are a new class of a polyphosphazene-platinum(II) conjugate antitumor agent having high tumor tissue selectivity due to its enhanced permeability and retention effect to tumor tissues, and a preparation method thereof: ##STR1##

wherein x represents the number of the ethylene oxide repeating unit and is 7, 12 or 16; y is 0, 1 or 2; z represents molar content of polyethylene glycol in the range of 0.5-1.5; n represents degree of polymerization of polyphosphazene in the range of 30-100; and A-A represents a diamine.