Through the rotation field experiment with pineapple and sugarcane as the crop from the year 2000 to 2002, the effect of pineapple and sugarcane rotation on the ecosystem of the soil was been studied.The results showed that: compared with the continuous cropping of sugarcane-sugarcane field, the crop rotation of Pineapple-sugarcane could improve the capability of aeration and water storage, accelerate the release of the quick effective nutrient, strengthen soil enzyme activity and increase the total quantity of microbe of soil (general aerobic bacteria, fungi, actinomyce). The beneficial ammonifier and the nitrifying bacteria were doubled, while the unbeneficial anaerobic bacteria, denitrify bacteria were inhibited, the quantity of nitrogen-fixing bacteria was less and not changed more. The nature of sugarcane soil ecosystem improvement affected by the rotation of pineapple and sugarcane lies in that after the rotation of pineapple-sugarcane, because of different crops, leading to the improvement of soil structure and the increase species and number of soil micro-organism, and the increase of soil biological activity, and the increase of speedy nutrients.