To the problem of spaceborne multi-spectral image compression，a compression scheme based on sub-band spectral transform is proposed. First，two-dimensional discrete wavelet transform is performed to each band of multi-spectral image sequences for removing the spatial correlation.Subsequently，in order to remove the spectral correlation，each level of the sub-bands resulted from wavelet transform are regarded as a whole，and sub-band spectral KLT is performed on the two bands by using serial pairwise transform. Finally，Embedded Block Coding with Optimized Truncation(EBCOT)algorithm is performed on all principal components to realize optimal rate-distortion compression. Experimental results show that the proposed algorithm not only provides better compression performance，but also has lower encoding complexity，which is suitable for the onboard compression of multi-spectral images.