This paper presents a reversible or lossless watermarking algorithm base on parity for medical image. Compared to the general image medical image have more close to the 0-value pixels which are easy to overflow in the embedding process. By embedding test, select pixels which are able to carry information to embed watermarking information. This algorithm takes full advantage of all pixels which are able to carry watermark information, so the embedded rate of the image is enhanced. To ensure the correct classification of pixels before extracting and the reversibility of the algorithm, the testing algorithm based on the parity of data is improved. Experiments show that this algorithm can both improve the watermarking capacity of the image and guarantee visual effect. For general images, both watermark embedding and visual effect have some improvement.