Disclosed is an attenuation method of an influenza virus, that is, a reassortant virus and a preparation method thereof. The disclosed reassortant virus has, in a ratio of 6:2, genes encoding a recombinant non-toxic protein and a wild type non-toxic protein, and genes encoding toxic proteins, HA (hemagglutinin) and NA (neuraminidase), of an influenza virus, the recombinant non-toxic protein consisting of a substituted caspase recognition sequence without a change of a protein size within the wild type non-toxic protein of the influenza virus. The disclosed attenuated influenza virus shows a high attenuation without a reduction of productivity in a fertilized egg. Accordingly, the method can be used as an economically efficient live vaccine preparation method which has both safety and efficiency and can use a fertilized egg as a production system. Also, since a protein is not removed or modified during attenuation, the method can be used in combination with a conventional attenuated vaccine preparation technology such as cold-adaptation.