Disadvantages of wavelet approaches were found by the authors as follows. When the scale of wavelet decomposition is small, mosaic occurs on the fusion image; when the scale of wavelet decomposition is large, color content of fusion image loses greatly, and when the scale of wavelet decomposition is equal to image size, mosaic disappears and color content degrades mostly. It was also found that the mosaic size is equal to the scale. The larger the scale is, the more degradation of color content will be. The reason of the above disadvantages was analyzed carefully based on one-dimensional signal model. Finally, experiments were given to illustrate the disadvantages of wavelet approaches.