A safety syringe which comprises: a barrel, extended in a direction of an axis, and including a first tube portion, a second tube portion extended from an end of the first tube portion, and a first folded ring including a movable first flexible portion and flexibly branched and formed on an inner side of the first tube portion and disposed around the axis, such that after the first folded ring is unfolded, the first folded ring is bounded and extended from the first tube portion towards the second tube portion; a plunger, including a rod extended along the axis and movably inserted into the barrel, and a plug installed at an end of the rod; and a needle hub, detachably installed in the second tube portion, and the plug of the plunger being adapted to be snapped into the needle hub to stretch open the first flexible portion, so that the first flexible portion is separated from the needle hub, wherein the plunger can be moved to an injecting position and a disposal position with respect to the needle hub, and when the plunger is moved to the injecting position, the plug of the plunger is situated in the first tube portion and apart from the needle hub, and the first flexible portion pushes the needle hub to be fixed in the second tube portion, and when the plunger is moved to the disposal position, the plunger is situated away from the second tube portion, and the needle hub is released from the first folded ring and separated from the second tube portion, characterised in that the plunger can be moved to a limit position with respect to the needle hub, and when the plunger is moved to the limit position, the plug is snapped into the needle hub to stretch open the first flexible portion, so that the first flexible portion is separated from the needle hub, and the second tube portion of the barrel has an internal diameter smaller than the internal diameter of the first tube portion of the barrel, and the barrel further has a storage space formed between the first tube portion and the second tube portion for accommodating the first flexible portion, and at the limit position, the first flexible portion is pushed by the plug into the storage space. a barrel, extended in a direction of an axis, and including a first tube portion, a second tube portion extended from an end of the first tube portion, and a first folded ring including a movable first flexible portion and flexibly branched and formed on an inner side of the first tube portion and disposed around the axis, such that after the first folded ring is unfolded, the first folded ring is bounded and extended from the first tube portion towards the second tube portion; a plunger, including a rod extended along the axis and movably inserted into the barrel, and a plug installed at an end of the rod; and a needle hub, detachably installed in the second tube portion, and the plug of the plunger being adapted to be snapped into the needle hub to stretch open the first flexible portion, so that the first flexible portion is separated from the needle hub, wherein the plunger can be moved to an injecting position and a disposal position with respect to the needle hub, and when the plunger is moved to the injecting position, the plug of the plunger is situated in the first tube portion and apart from the needle hub, and the first flexible portion pushes the needle hub to be fixed in the second tube portion, and when the plunger is moved to the disposal position, the plunger is situated away from the second tube portion, and the needle hub is released from the first folded ring and separated from the second tube portion, characterised in that the plunger can be moved to a limit position with respect to the needle hub, and when the plunger is moved to the limit position, the plug is snapped into the needle hub to stretch open the first flexible portion, so that the first flexible portion is separated from the needle hub, and the second tube portion of the barrel has an internal diameter smaller than the internal diameter of the first tube portion of the barrel, and the barrel further has a storage space formed between the first tube portion and the second tube portion for accommodating the first flexible portion, and at the limit position, the first flexible portion is pushed by the plug into the storage space.