This paper studies the vein extraction technique based on the susceptibility weighted imaging (SWI) and introduced an improved self-adaptive threshold method based on the vessel enhancing diffusion. The approach employs the combination indicator of the local gray character, the global gray character and the tubular information of the vein. It first applies the vessel enhancing diffusion filter to enhance the continuity of the vein, increases the detection rate of tiny vein and suppresses the nucleus areas. And then it uses the improved self-adaptive threshold method to extract the vein. The results demonstrate that this approach can solve the problem above and extract the vein from the SWI image accurately.