An novel idea that image fusion should be performed at similar scale is proposed. Based on the idea “similar scale”, an image fusion algorithm is presented. At first, the “à trous” discrete wavelet transform is used to decompose the panchromatic image so that its approximation and the interpolated multispectral image have the similar scale. Secondly, image fusion based on weighted multiscale fundamental form (WMFF) is performed on them to obtain the synthesized approximation. Finally, the inverse “à trous” wavelet transform of the synthesized approximation and the detail of the panchromatic image reconstructs a high spatial resolution multispectral image. Compared with other wavelet-based techniques in the literature, the proposed image fusion algorithm has better performance.