The invention discloses a one-way torque adjustable screwdriver which comprises a body and a polygonal elastic member disposed inside the body. A torque transmission device and a torque adjusting device are mounted on two ends of the polygonal elastic member, respectively. A drive rod is connected to the torque transmission device. Driving the torque adjusting device to move can change a length ofthe polygonal elastic member, thus change an acting force of the polygonal elastic member exerted on the torque transmission device, change an output torque provided by the drive rod, and thus achieve the effect of easy operation. The torque adjustable screwdriver is capable of entering an idle state when an output torque of the screwdriver reaches a predetermined value in order to prevent the over-torque action from being caused on the screwing member. The torque adjustable screwdriver has the effect of simplifying the adjustment of an output torque by rotating a torque adjusting device.