IFN-alpha mutants are obtained by substituting Cys for Tyr at position 85 or 86 in existing IFN-alpha. Their polyethylene glycol derivatives with high in vitro antiviral activity and prolonged in vivo half-life are also provided, wherein a polyethylene glycol moiety is covalently bound to the free Cys residue of an IFN-alpha mutant. The preparation methods of PEG derivatives of IFN-alpha mutants and medical compositions comprising the derivatives are also provided. The test results showed that the IFN-alpha mutants of the present invention are ready to prepare and have high activity; their polyethylene glycol derivatives have extended lifetime in the body and low clearance rate.