Hydrolyzed polymaleic anhydride (HPMA) plays an important role in improving soil, promoting seed germination and growth, but the further mechanism of its interaction with soil are still unclear. Thus in this paper HPMA was used to treat the colloids of dark-brown forest soils and degraded saline-alkali grassland soil, and then Laser particle size analyzer, Atomic force microscope (AFM), Transmission electron microscope (TEM), X-ray powder diffraction(XRD), Infrared spectrum (IR) and X-ray photoelectron spectroscopy (XPS) were used to study the surface structures, mineral crystals and organic compositions of the colloids in order to reveal the further mechanism of the differences in HPMA effects on different soil colloids. The results showed that the improvement of HPMA on soil degradation is related with its influence on the surface structures, mineral crystals and organic compositions of soil colloids, but the remarkable difference of the influences existed between forest dark brown soil and degraded saline-alkali grassland soil. For dark brown soil forest, the crystallinity of minerals decreased, but no strong chemical reaction happened, more functional groups tended to increase, while carbonate functional group decreased only by 3% – 8%. The colloid size increased, the surface ones increased by 13.62% (P>0.05) while the deep ones by 34.96% (P<0.05). For degraded saline-alkali grassland soil, colloid size increased significantly (27.54%, P<0.05), in addition, quartz crystallinity decreased by 7.10% and the others by 30% more, which led to the significant increases of all soil mineral grains in size (increased by 101.37%, 56.16% and 50.76% for hydromica, vermiculite and kaolinite, respectively), with the contents of more functional groups reduced, among of which carbonate functional groups reduced by 96.79%. This variations result in element ratios of high quality soils (C︰O = 0.27; C︰N 9.98; C︰Si = 0.91; O︰Si = 3.38; Si︰N = 10.93), which indicate that strong chemical reactions happened between HPMA and calcium carbonate and other minerals in soil colloids.