Based on the analysis of composition for agricultural production factors, this paper compares the difference between proportions for agricultural production factor input of 29 provinces of China in 1985 and 2004, and then examines the difference of output elasticity for every factor over two periods by econometric models. The results show that with the development of market, the proportion of input for land and labor is decreasing, while that of capital is increasing. As for as the output elasticity is concerned, the highest is that of capital, followed by those of labor and land in turn in 2004. Compared with those of 1985, the output elasticity for capital, land and labor changed differentially, during which the first two declined in different scales, while the last increased. The findings also show that Chinese agriculture is experiencing the period of increasing margin for factor inputs, and it is of great importance to increase the input of capital in the near future.