The invention provides an automatic extraction method for the vertebra in MRI lumbar vertebra image. The invention relates to the field of medical image processing technology and more particularly to an automatic extraction method for the vertebra in MRI (Magnetic resonance imaging) lumbar vertebra image. With the progress in medical image technology, MRI has already become an important means for diagnosing lumbar vertebra troubles. Traditionally, it takes about ten minutes to locate a troubled vertebra of a patient through the use of a mouse, which directly influences the working efficiency of a doctor to diagnose. The invention solves the problem of automatic identification and extraction of vertebra in MRI lumbar vertebra image. Through the use of the MRI lumbar vertebra's T2 image, it is possible to automatically extract the vertebra and to automatically calculate the size, the width, the height and the average grayscale of the vertebra so as to realize quantitative diagnosis of vertebra troubles rapidly and to increase the diagnosing efficiency of vertebra diseases.