Changes in agricultural population can significantly influence the progress of mechanization, which provides efficient momentum to the further development of agriculture. We examine the effect of the declining agricultural population on mechanization and determine the variables that have decisive power over mechanical adoption decisions. This analysis of a panel data of top six out-migration provinces in China under a fixed effect model, we find that the overly fast and unusual decline in agricultural population actually slowed down the progress of mechanization, and the many years of large scale out-migration encouraged by government actually jeopardized sustainable agricultural development. Results underscore the need for considerable attention on the growth of annual farm incomes and agricultural products import that could have substantial effects on agricultural mechanization decisions.