According to the data from Investigation Report of Land Use Change in China, The Land Resources Communique of China and Chronicle of Statistical Data for Five Decades of New China issued from Ministry of Land Resources, we select two indices: change of farmland quantity and policy of farmland protection. According to econometric theory, by using Eviewes 5.1 software, co-integration analysis, Granger causality test, impulse response and other analysis methods, we analyze the relationship between change of farmland quantity and policy of farmland protection in China since the reform and opening-up. The results show that there is long-term balanced relationship between change of farmland quantity and policy of farmland protection, and there is a certain mechanism restricting motion of variables between the two so as to make the two deviate from each other little and step towards balance in the long run; there is unilateral causality relationship between farmland change and policy of farmland protection, namely that the farmland change is the Granger cause of policy of farmland protection, while policy of farmland protection is not the Granger cause of farmland change; impulse response and variance decomposition indicate that farmland change plays the role of promoting policy of farmland protection continuously, and the role is strengthened along with prolonged lag period; the policy of farmland protection has strong inertia, because it is impacted by the former level of itself, and the policy of farmland protection plays insignificant role in promoting farmland quantity. Consequently, the important approach of solving problem of rapid decrease of farmland is to formulate long-term strategy, strengthen theoretical research of farmland protection and reinforce degree of formulation, implementation and surveillance of farmland protection policy.