The casing damage has been a big problem in oilfield production. The current detection methods mostly are used after casing damage, which is not very effective. With the rapid development of China’s offshore oil industry, the number of offshore oil wells is becoming larger and larger. Because the cost of offshore oil well is very high, the casing damage will cause huge economic losses. What’s more, it can also bring serious pollution to marine environment. So the effective methods of detecting casing damage are required badly. The accumulation of stress is the main reason for the casing damage. Magnetic anisotropy technique based on counter magnetostriction effect can detect the stress of casing in real time and help us to find out the hidden dangers in time. It is essential for us to prevent the casing damage from occurring. However, such technique is still in the development stage. Previous studies mostly got the relationship between stress and magnetic signals by physical experiment, and the study of physical mechanism in relative magnetic permeability connecting the stress and magnetic signals is rarely reported. The present paper uses the ANSYS to do the three-dimensional finite element numerical simulation to study how the relative magnetic permeability works for the oil casing model. We find that the quantitative relationship between the stress’ s variation and magnetic induction intensity’s variation is: Δδ =K* ΔB, K = 8.04×109, which is proved correct by physical experiment. The casing damage has been a big problem in oilfield production. The current detection methods mostly are after casing damage, which is not very effective. With the rapid development of China’s offshore oil industry, the number of offshore oil wells is becoming larger Because the cost of offshore oil well is very high, the casing damage will cause huge economic losses. What’s more, it can also bring serious pollution to marine environment. is the main reason for the casing damage. It is essential for us to prevent the casing damage from occurring However, such technique is still in the development stage. Previous studies mostly got the relationship between stress and magnetic signals by physical experiment , and the study of physical mechanism in relative magnetic permeability connecting the stress and magnetic signals is rarely reported. The present paper uses the ANSYS to do the three-dimensional finite element numerical simulation to study how the relative magnetic permeability works for the oil casing model . We find that the quantitative relationship between the stress’ s variation and magnetic induction intensity’s variation is: Δδ = K * ΔB, K = 8.04 × 109, which is proved correct by physical experiment.