The medical image fusion technology has become a hot research field of medical imaging; the image segmentation is an important task in image fusion. Due to the diversity and complexity of the medical images, the medical image fusion technology has a big difficulty in image segmentation. Threshold method becomes an important image segmentation way due to its high efficiency and simple feature. However, for the segmentation of the complex medical images, the effect of the threshold method is far from being ideal. Powell algorithm is the best direct search way; the application of the improved Powell algorithm can search the target better. Therefore, the authors propose an image segmentation method that is based on variable threshold and combines with Powell algorithm. Through the simulation experiment, this method can segment the images rapidly and effectively, and features a strong robustness.