A safe syringe with automatically retractable needle comprises a cylinder with a holding portion, a needle seat which is fixed within the front end of the cylinder via the holding portion and has a needle holder centrally and a plurality of ribs laterally, and a pusher plunger having a chamber with negative pressure and a constricted opening on the front end thereof. On the edge of the constricted opening is formed a cutting part. And a piston is provided to cover the opening of the pusher plunger. When the pusher plunger is pushed toward the front end of the cylinder, the piston will be cut out and the holding portion will be compressed and deformed to release the needle seat so that the chamber with negative pressure communicates with outside and the detached needle seat can be drawn into the plunger together with a needle by the pressure difference.