The present invention relates to a new class of cationic linear polyphosphazenes bearing as side groups a hydrophilic poly(ethylene glycol) and a spacer group selected from the group consisting of lysine, oligopeptides containing lysine, amino-ethanol, amino-propanol, amino-butanol, amino-pentanol and amino-hexanol, and the polyphosphazene-drug conjugates comprising hydrophobic anticancer drugs by covalent bonding and the preparation methods thereof. The present polyphosphazene-drug conjugates exhibit outstanding tumor selectivity and low toxicity.