An electronic cylinder for a door lock comprising: an exterior knob; an exterior cylinder core connected securely to an end of said exterior knob; a motor disposed within said exterior cylinder core, having a protrusion block and an axle extending axially and outwardly from said protrusion block; a clutch unit connected operably to said motor; a cam member associated with said clutch unit and said motor for driving a latch assembly to lock or unlock the door upon causing rotation between said clutch unit and said motor; a cylindrical coupler seat disposed between said motor and said clutch unit, characterised in the cylindrical coupler seat including a rear chamber for receiving said clutch unit and a front portion located opposite to said rear chamber and having a front end face provided with a blocking opening constituted by an injection channel indented inwardly from said front end face and extending inwardly and radially from a first point of a periphery confining said front end face, an recess indented inwardly and axially from a bottom surface of said injection channel and an outflow channel indented inwardly from said front end face and extending inwardly and radially from a second point of the periphery opposite to said first point to terminate at said recess, said bottom surface of said recess being formed with a through hole permitting extension of said axle for operably engaging said clutch unit, said front portion further having a preventing block formed between said bottom surface of said recess and said outflow channel, said preventing block being located in a depth with respect to said front end face complementing with said protrusion block of said motor such that once said protrusion block is seated in said recess, said front end face of said coupler seat abuts fittingly against said motor around said protrusion block; and in comprising a waterproof filler hermetically filling said recess in said coupler seat. an exterior knob; an exterior cylinder core connected securely to an end of said exterior knob; a motor disposed within said exterior cylinder core, having a protrusion block and an axle extending axially and outwardly from said protrusion block; a clutch unit connected operably to said motor; a cam member associated with said clutch unit and said motor for driving a latch assembly to lock or unlock the door upon causing rotation between said clutch unit and said motor; a cylindrical coupler seat disposed between said motor and said clutch unit, characterised in the cylindrical coupler seat including a rear chamber for receiving said clutch unit and a front portion located opposite to said rear chamber and having a front end face provided with a blocking opening constituted by an injection channel indented inwardly from said front end face and extending inwardly and radially from a first point of a periphery confining said front end face, an recess indented inwardly and axially from a bottom surface of said injection channel and an outflow channel indented inwardly from said front end face and extending inwardly and radially from a second point of the periphery opposite to said first point to terminate at said recess, said bottom surface of said recess being formed with a through hole permitting extension of said axle for operably engaging said clutch unit, said front portion further having a preventing block formed between said bottom surface of said recess and said outflow channel, said preventing block being located in a depth with respect to said front end face complementing with said protrusion block of said motor such that once said protrusion block is seated in said recess, said front end face of said coupler seat abuts fittingly against said motor around said protrusion block; and in comprising a waterproof filler hermetically filling said recess in said coupler seat.