The circular cross section beam is widely used in mechanical equipment and civil engineering. The existence of cracks directly endangers safety and reliability of beam. A crack identification method based on frequency contour line was explored.The evaluations of stress intensity factors for a beam subjected to tension, bending moment and shear were deduced by using a new method, and the evaluations of corresponding flexibility coefficient were given. According to FEM, the compliance matrices of cracked and uncracked element beam were derived, the motion differential equation for free vibration was given, and the vibration characteristic of the beam with crack was discussed. By means of the influence of the crack parameters on the natural frequency, the natural frequency was used as a detector, and the crack was identified by the natural frequency contour line method, the method was improved based on experiments. The results show that the improved frequency contour method makes up for the deficiency that there is no intersection or triangle in some conditions, making it more feasible in engineering practice, and the maximum error of the relative crack depth is 77.5%, the maximum error of the relative crack location is only 12.6%.