This paper presents a new approach based on the method of magnetic flux leakage (MFL) for the one-sided butt weld (OSBW) in order to detect and identify the weld defect. Combining the numerical analysis and experimental research method, a new non-contact scanning MFL system (NCSMFLS) for the weld defect is designed and implemented whose magnetization direction is perpendicular to marching direction. Then NCSMFLS is used continuously in the weld defect such as the rectangular slot in the weld and the rectangular slot in the weld heat-affected zone by the rectangular slot defect to simulate the crack defect, afterwards, this paper reviews the characteristics such as size and distribution of leakage magnetic field (LMF) of the above-mentioned two kinds of positions. Also, the fitting curve by least squares method and polynomial regression equation for two kinds of positions are obtained. The outcomes indicate that the numerical simulation results and experimental results match each other well and their correlation coefficient of the original data: rectangular slot in the weld Rwr=0.9764; rectangular slot in the weld heat-affected zone Rhr=0.943, furthermore, the results verify the feasibility and the validity for the suggested detection system.