Abstract: For studying the patial defect phenomenon of cement sheath caused by low displacement efficiency and gas channeling in the cementing process, the finite element method is used to establish different size and different orientation models of cement sheath defect and to simulate the propagation rules of ultrasound in multilayer medium of casing well. By extracting the echo amplitude and the headwave time in different directions and by analyzing ccho frequency spectrum, the time domain and frequency domain characteristics of the models are compared effectively. For determining the cement sheath defect at different orientations, the defect's sensitive frequency (Dev) is introduced, and it is found that a great value of Dev in the defective position appears under a certain excitation frequency and the defects in different directions show different Devs, which can determine the orientation of the defect.