Self-tightening adjustable spanner

The invention provides a self-tightening adjustable spanner comprising an upper jaw and a lower jaw. The lower jaw is provided with a friction block which is hinged to the lower jaw. The lower jaw is provided with an installing notch sunken towards the lower jaw. The friction block is installed in the installing notch. The upper surface of the friction block and the lower surface of the upper jaw form a jaw tightening face which is provided with anti-slip protrusions. According to the self-tightening adjustable spanner, the acting force is changed into huge friction force from pressure, the huge friction force acts on a whole face opposite to a nut, the friction force is perpendicular to a rotation direction, and therefore larger torque can be achieved; in a same way, the manner that the force acts on one straight line is changed into the manner that the force acts on the whole face, damaging force is shared, the damaging force borne by each unit area is remarkably reduced, and it is guaranteed that the nut is not damaged and an operator is protected; and positive pressure needed by the friction force is provided by sway bracing resultant force.