A cable tie tensioning and severing tool, comprising: a housing having an end provided with an inlet and an opposite end provided with a connection opening, wherein the inlet is an entrance for a cable tie; a cutting member pivotally provided in the housing, wherein the cutting member has a front end and a rear end defined respectively on two opposite sides of a pivotally connected portion of the cutting member, and the front end is provided with a blade for severing the cable tie; a pulling member slidably provided in a channel in the housing, wherein the pulling member is provided with a toothed block for pulling the cable tie and is pivotally provided with a pushing block abutting against the rear end of the cutting member; a trigger pivotally connected to the housing, wherein the trigger is provided with a stop block abutting against the pushing block wherein by pivoting the trigger, the stop block is adapted to be driven to push the pushing block such that the pushing block pulls the cable tie through the pulling member and, once a pressing force is overcome, the pushing block is adapted to push and thereby pivot the cutting member to sever the cable tie; an adjustment mechanism comprising a pressing hammer pivotally provided in the housing, a turning knob pivotally provided in the connection opening, characterized in that the adjustment mechanism further comprises an elastic member provided between the pressing hammer and the turning knob, wherein the elastic member has one end directly pressing against a ramped end face of the turning knob, the ramped end face has a height changing continuously in a spiral direction, the elastic member has an opposite end directly pressing against the pressing hammer such that the pressing hammer is adapted to apply a pressing force to the cutting member, the elastic member is adapted to press against the ramped end face at a position adjustable by rotating the turning knob, and the magnitude of the pressing force depends on the position at which the elastic member presses against the ramped end face; wherein the turning knob is provided with an axle, the ramped end face extends spirally around a center defined by the axle, the axle is peripherally provided with a plurality of positioning grooves, and the housing is provided with an elastic positioning member configured to be elastically engaged in any one of the positioning grooves of the axle and thereby secure the turning knob in position. a housing having an end provided with an inlet and an opposite end provided with a connection opening, wherein the inlet is an entrance for a cable tie; a cutting member pivotally provided in the housing, wherein the cutting member has a front end and a rear end defined respectively on two opposite sides of a pivotally connected portion of the cutting member, and the front end is provided with a blade for severing the cable tie; a pulling member slidably provided in a channel in the housing, wherein the pulling member is provided with a toothed block for pulling the cable tie and is pivotally provided with a pushing block abutting against the rear end of the cutting member; a trigger pivotally connected to the housing, wherein the trigger is provided with a stop block abutting against the pushing block wherein by pivoting the trigger, the stop block is adapted to be driven to push the pushing block such that the pushing block pulls the cable tie through the pulling member and, once a pressing force is overcome, the pushing block is adapted to push and thereby pivot the cutting member to sever the cable tie; an adjustment mechanism comprising a pressing hammer pivotally provided in the housing, a turning knob pivotally provided in the connection opening, characterized in that the adjustment mechanism further comprises an elastic member provided between the pressing hammer and the turning knob, wherein the elastic member has one end directly pressing against a ramped end face of the turning knob, the ramped end face has a height changing continuously in a spiral direction, the elastic member has an opposite end directly pressing against the pressing hammer such that the pressing hammer is adapted to apply a pressing force to the cutting member, the elastic member is adapted to press against the ramped end face at a position adjustable by rotating the turning knob, and the magnitude of the pressing force depends on the position at which the elastic member presses against the ramped end face; wherein the turning knob is provided with an axle, the ramped end face extends spirally around a center defined by the axle, the axle is peripherally provided with a plurality of positioning grooves, and the housing is provided with an elastic positioning member configured to be elastically engaged in any one of the positioning grooves of the axle and thereby secure the turning knob in position.