The research was conducted in high-mountain of Datong County in Qinghai Province,where is a demonstration site for Land Conversion Program(LCP).For the specific converted lands,the typical soil type-Chernozem was selected as the main research object.Using the idea of substituting space for time,statistic analysis method and Principal Component Analysis(PCA),the paper appraises and compares soil quality index quantitatively so as to further study soil quality evolution in the time series.Results indicate that soil quality index in the 30 year conversion fluctuates with time,showing a rising trend.The history of the conversion can be basically divided into three stages of the initial,middle and later periods.The initial period is the first 5 years after the conversion initiation,which is characterized by the sapling stage and a relative increase in soil quality index.The middle period ranges from 5 to 20 years after land conversion initiation.Conifer and broad-leaf forests grow rapidly in the period,and soil quality index increases greatly and reaches its peak.The later period is from 20 years after conversion initiation to current time.The period is the stable period characterized by the forest with dense canopy and a modest decline in soil quality index.After conversion for more than 25 years,forests will self-sparse,and soil quality index will rise to some extent and tend to be stable.