Remote sensing image fusion (RSIF) can generate an integrated image with high spatial and spectral resolution. The fused remote sensing image is conducive to applications including disaster monitoring, ecological environment investigation, and dynamic monitoring. However, most existing deep learning-based RSIF methods require ground truths (or reference images) to train a model, and the acquisition of ground truths is a difficult problem. To address this, we propose a semi-supervised RSIF method based on the multi-scale conditional generative adversarial networks (cGANs) by combining the multi-skip connection and pseudo-Siamese structure. This new method can simultaneously extract the features of panchromatic and multi-spectral images to fuse them without a ground truth; the adopted multi-skip connection contributes to presenting image details. In addition, we propose a composite loss function, which combines the least squares loss, L1 loss, and peak signal-noise ratio loss to train the model; the composite loss function can help to retain the spatial details and spectral information of the source images. Moreover, we verify the proposed method by extensive experiments, and the results show that the new method can achieve outstanding performance without relying on the ground truth. Index Terms-conditional generative adversarial network; deep learning; image fusion; loss function; remote sensing image fusion I. INTRODUCTION MAGE fusion aims to fuse the complementary information in two or more source images obtained by different sensors so that a new comprehensive image can be generated [1][2][3]. The fused image can present more useful information than any one of the source images. Therefore, image fusion techniques can provide a high-quality fused image to help interpret a scene or target, and related research areas include medical images [5], multi-focus images [3][6], infrared and visible images [7][8], and remote sensing images [9][10][10].Remote sensing images with high spatial and spectral resolution play an important role in geological exploration,