The changes and mechanism of soil fertility under two kinds of sugarcane cultivations during 25a were studied at Longchuan farm based on the data obtained from the surveys in 1983 and 2007. The results showed as follows: soil organic matter contents in dry land and paddy field reduced by 12.65% and 25% respectively, total nitrogen content changed little in dry land but reduced by 1.42% in paddy field, available N increased by 1.53% in dry land but reduced by 22.65% in paddy field, available P increased by 95.02% and 6.02% respectively in dry land and paddy field, available K increased by 26.36% in dry land but reduced by 47.28% in paddy field. The data of sugar yield and sugar recovered ratio proved that sugarcane leaf returning to field could improve soil fertility sustainably, which was attributed to for the continuous high sugar yield in studied region.