A safe disposable injector with a changeable and automatically retractable needle includes a needle cylinder (1) and a plunger (2), and also includes a medicament-dispensing needle unit (3) and an injecting needle unit (4). The medicament-dispensing needle unit (3) and the injecting needle unit (4) are changed according to the requirement in use. The present invention will prevent harmful effects caused by needle deformation and variation of smoothness and sharpness during the injection to the human body. The operation of the present invention is the same as the convention injection operation. The present invention is convenient to use and reliable to operate. After injection, the injector needle can be retracted by an automatic retraction mechanism to avoid the accidental hurt to medical staff and patients by a polluted needle tip. The present invention is suitable for automatic production on the assembly line.
