A needle cylinder is formed by connecting a front cylinder body and a rear cylinder body in a sealed mode, a front end sealing cap is detachably installed on a Luer taper at the front end of the front cylinder body, and a push rod
piston and a middle sealing
piston are sequentially arranged in a needle cylinder cavity in a sliding mode. The middle sealing
piston divides the needle cylinder cavity into a liquid
medicine storage cavity and a
powder storage cavity, a bypass structural surface with a bypass groove is formed on the inner wall of the needle cylinder cavity, an axial blocking convex rib is arranged in the middle of the bypass structural surface, and in the liquid
medicine mixing stage, the middle sealing piston is blocked by the axial blocking convex rib and kept positioned; the middle sealing piston can respond to the thrust of the push rod piston,
cross over the blocking convex rib and enter a
medicine injection stage, the front end sealing cap comprises a shell, one end of an
assembly shell part of the shell is provided with an
insertion part, the other end of the
assembly shell part is provided with an exhaust port opened and closed through a sealing end cover, and a
filter element component for achieving liquid stopping and exhaust is arranged in an installation cavity of the
assembly shell part. The medicine mixing operation is safe, reliable and convenient.