Abstract: Magnetic powder detection is an important method for work-piece superficial crack detection. Traditional magnetic powder crack detection is manpower consuming, time consuming, high expenses, low precision, and fallibility. Modern industrial detection technology requires work-piece crack auto-detection. Because of exterior status, veritable or feigned crack object, site condition, etc., existing method can not successfully auto-detect and identify work-piece cracks. Fluorescent magnetic powder image and crack image characteristics are analyzed, morphological grads arithmetic operators image fringe detection based on sub-area threshold is studied, crack identification arithmetic based on Fisher linear discrimination is designed according to long-width ratio and round shape degree character. Cart-wheel-axis crack detection line equipped with this auto-detection and identification technology got an efficient crack detection ratio as high as 90%.