As a deep learning network with an encoder-decoder architecture, UNet and its series of improved versions have been widely used in medical image segmentation with great applications. However, when used to segment targets in 3D medical images such as magnetic resonance imaging (MRI), computed tomography (CT), these models do not model the relevance of images in vertical space, resulting in poor accurate analysis of consecutive slices of the same patient. On the other hand, the large amount of detail lost during the encoding process makes these models incapable of segmenting small-scale tumor targets. Aiming at the scene of small-scale target segmentation in 3D medical images, a fully new neural network model SUNet++ is proposed on the basis of UNet and UNet++. SUNet++ improves the existing models mainly in three aspects: 1) the modeling strategy of slice superposition is used to thoroughly excavate the three dimensional information of the data; 2) by adding an attention mechanism during the decoding process, small scale targets in the picture are retained and amplified; 3) in the up-sampling process, the transposed convolution operation is used to further enhance the effect of the model. In order to verify the effect of the model, we collected and produced a dataset of hyperintensity MRI liver-stage images containing over 400 cases of liver nodules. Experimental results on both public and proprietary datasets demonstrate the superiority of SUNet++ in small-scale target segmentation of three-dimensional medical images.