During the highspeed magnetic flux leakage testing, the effective magnetization time decreases with the testing speed increase. It may lead to the nonmagnetic saturation of the measured component. The magnetization effectiveness could be affected. To solve this problem, the square wave excitation is used to simulate the external magnetic field transients. The internal magnetic field response model of the steel pipe under the transient magnetic field is formulated. The formulation process and influencing factors of the internal saturation field of the steel pipe are studied. The features of the defect leakage magnetic field during highspeed magnetic flux leakage detection are analyzed. By using finite element analysis, the influences of magnetic field intensity and steel pipe material on magnetization lag time and defect detection are calculated. The highspeed magnetic flux leakage testing platform is designed. Experimental research on the steel pipe defects under different operating speeds and external magnetic field strengths are investigated. Results show that the central magnetic field of the inner wall of the steel pipe obviously lags behind the external magnetic field when the external magnetic field transients. The internal saturation field of the steel pipe is related to the magnetic field strength and the material conductivity. When the intensity of the external magnetic field is increased, the saturation field can be quickly established. Meanwhile, the defect detection effect and magnetic flux leakage detection speed can be improved. There is a good consistence between experiment results and theoretical analysis.