An injection device has a dose controlling assembly, an operation assembly, an injection pushing assembly, and a clutch assembly. The dose controlling assembly has an outer tube, a dose controlling tube, a driving tube, a connection tube, and a dose controlling collar. The driving tube has a limiting portion. The dose controlling collar is mounted in the dose controlling tube and has a spiral limiting portion selectively engaged with the limiting portion of the driving tube. The operation assembly is mounted on a rear end of the dose controlling assembly. The injection pushing assembly is mounted on a front end of the dose controlling assembly. The clutch assembly is mounted in a front segment of the outer tube and is connected with the injection pushing assembly.
