A safety syringe includes an outer barrel, a needle unit, an outer barrel plug, an inner barrel, an inner barrel plug, a needle clamper, and a vacuum generating device. The needle unit is disposed within the front end of the outer barrel. The outer barrel plug is disposed within the outer barrel; the outer barrel plug is connected with the needle unit so as to fix the needle unit. The inner barrel plug is disposed within the front end of the inner barrel. The needle clamper is connected with the inner barrel plug. The needle clamper is able to clamp a rear opening of the needle unit. The vacuum generating device is disposed within the inner barrel, and the needle unit can be retracted into the inner barrel by the low pressure which is generated by the vacuum generating device.