With the implementation of the ecological migration project in karst area of Guangxi Zhuang Autonomous Region, a large number of grasslands were reclaimed for cultivated fields in immigration area, and land reclamation inevitably affected the the soil fertility. By choosing corn land of 30 years and 15 years and grassland as the research objects, we studied the influence of grassland reclamation on soil fertility based on the theory of space for time. Soil fertility was evaluated on the basis of determining weight of factors by means of the correlation coefficient and normalization of factors through membership function. The results showed that after grassland replaced by corn field, the soil fertility declined in 0-100 cm soil layer; The degradation of 30 years corn was less serious than that of 15 years corn, so the soil fertility of 30 years corn was better; The degradation of soil fertility related to soil depth; The degradation was most serious at the beginning of reclamation because of the interference of human activities, but soil fertility was restored with the extension of cultivation.