Abstract: 【OBJECTIVE】 Based on theory of sustainable development of cotton field, the bonsai and the field experiments have been carried out to discover the effect of water retaining agent on soil and breading to the growth of cotton. 【METHOD】 The methods of ninhydrin and molybdenum blue, and bi directional soil section slicing were used. 【RESULTS】 The experiment results showed that: When content of the polymer in soil was in the range of 0.030%-0.100%, evaporation of soil moisture deceased and the pH value tended to be lowered The study showed that mulching could quicken the growth and development of cotton roots during early growth period, which reflected in the enhanced root visor and the root dry weight per plant. The distribution of root systems in soil was comparatively reasonable, and deep roots decreased slowly, which contributed to the proportional development of cotton plants, and prevented cotton plants from premature senescence. 【CONCLUSION】 The amount used in 30-45kg/hm-2 could increase the output of the cotton, obviously the production increase scope reaches above 10%, to the textile fiber quality not tremendous influence, the bell number increases 5.7~9.4 ten thousand per /hm2, the bell increases 0.1~0.19g.