Abstract: An automatic injection pen includes a shell, a needle cover slidably provided in the shell, an inner sleeve, a pen cap, a spring cylinder, a pen tube stopper and a syringe assembly. The syringe assembly includes a barrel, a needle and a piston assembly. The piston assembly includes a push head, a push rod, a spring support portion, a locking protrusion, a guiding portion and an actuation portion from bottom to top. An outer side of the spring support portion is provided with a spring for drug administration. The piston assembly, the spring, the spring cylinder, the needle cover, and the pen tube stopper constitute a locking-actuation mechanism. A push rod of the piston assembly is provided with an elastic protrusion structure. A linear buffer is provided between the push head and a piston in the barrel to delay the needle withdrawal.