Image compression algorithms such as EZW and SPHT have a high compression rate.In order to they also have spatial scalability,SPIHT algorithm is analyzed.At the same compression rate,it is improved such that it has spatial scalability.In the proposed algorithm,the coding information of a isolated pixel(significant or insignificant) is outputted to the bitstream which the pixel is located;the coding information of a set is outputted to the bitstream which according to the highest layer pixels of the set is located.Although the low-frequency subband is encoded together with the high-frequency subbands,the experimental result show that the bit rate is very close to the results of the low-frequency subband is encoded independently.Meanwhile,an optimal bit allocation scheme between base-layer bitstream and enhanced-layer bitstream is also given in this paper.The simulation results show that the number of bit of base-layer bitstream is different from compressed images and the compression rate.