An electronic cylinder includes a knob; a cylinder core connected with the knob; a motor having a protrusion block; a clutch unit; a coupler including a chamber receiving the clutch unit and a front portion having a front end face with an opening constituted by an injection channel indented inwardly from the end face and extending from a first point of a periphery confining the end face, an recess indented inwardly from a bottom surface of the injection channel and an outflow channel indented inwardly from the end face and extending from a second point of the periphery, the bottom of the recess being formed with a hole permitting extension of an axle for engaging the clutch unit, a preventing block in a depth of the opening such that after assembly, the end face abuts against the motor; and a waterproof filler hermetically filling the recess.