Provided is a test sensor using a magnetostrictively induced guided wave based on an open magnetic circuit, comprising an excitation coil, a receiving coil and a magnetic device, the magnetic device comprises multiple test modules circumferentially and uniformly disposed thereon so as to be absorbed to the outer side of a to-be-tested slender component, each test module comprises a housing, a permanent magnet and a magnetic plate, two adjacent housings are connected to each other via an adjusting device, the excitation coil and the receiving coil are disposed in the vicinity of the test module, and are coaxially fit on the outer side of the to-be-tested slender component, the excitation coil operates to generate induced voltage in the receiving coil after the sinusoidal alternating current is input, and a computer can determine whether a defect occurs in the to-be-tested slender component after receiving the induced voltage. The sensor of the invention features simple structure, small size, light weight and convenient installation. Moreover, by serially connecting the sensor with multi-layered test coils disposed on both sides of the permanent magnet, it is possible to enhance an amplitude of a test signal, and to improve test sensitivity.