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Deep convolutional neural networks (CNNs) have been very successful in image denoising. However, with the growth of the depth of plain networks, CNNs may result in performance degradation. The lack of network depth leads to the limited ability of the network to extract image features and difficults to fuse the shallow image features into the deep image information. In this work, we propose an improved deep convolutional U-Net framework (RatUNet) for image denoising. RatUNet improves Unet as follows: (1) RatUNet uses the residual blocks of ResNet to deepen the network depth, so as to avoid the network performance saturation. (2) RatUNet improves the down-sampling method, which is conducive to extracting image features. (3) RatUNet improves the up-sampling method, which is used to restore image details. (4) RatUNet improves the skip-connection method of the U-Net network, which is used to fuse the shallow feature information into the deep image details, and it is more conducive to restore the clean image. (5) In order to better process the edge information of the image, RatUNet uses depthwise and polarized self-attention mechanism to guide a CNN for image denoising. Extensive experiments show that our RatUNet is more efficient and has better performance than existing state-of-the-art denoising methods, especially in SSIM metrics, the denoising effect of the RatUNet achieves very high performance. Visualization results show that the denoised image by RatUNet is smoother and sharper than other methods.