How reliable are cultivated land assets as old-age security for Chinese farmers is a scientific issue that needs to be urgently solved in the research field of land use change and rural pension system. To give a scientific judgment on the function of cultivated land as old-age security in the new era and provide a reference for improving the rural old-age security system, this paper regards the cultivated land with the contracting right as an asset for farm households, and systematically reveals the spatiotemporal evolution in the old-age security function of cultivated land asset from the national and provincial levels based on large-scale household survey data from the Rural Permanent Observation Sites (RPOS) conducted by the Ministry of Agriculture from 1986 to 2015. The results show that: (1) At the national level, compared with the constant price of 1986, the value of old-age security per capita cultivated land has dropped from 550 yuan to 150 yuan in the past 30 years, and the decline rate reached 72% during the study period. In the 1980s, cultivated land assets could meet the old-age expenditures of the elderly; but after 1994, the value of cultivated land as old-age security could not support the consumption of the elderly, and the gap in the demand for old-age security continued to expand. (2) At the interprovincial level, the value of cultivated land as the old-age security in all provincial-level regions was in a downward trend during the study period, and there were significant differences among provinces. In recent years, the contribution rate of the old-age security to the demand for old-age care has been declining, and the gap in the demand for old-age care has been expanding. The provinces along the Southeast Coast and the Yangtze River are the most prominent. Nowadays, cultivated land contract that began in the 1980s has lost its basic arguments, and the theory on social protection of cultivated land needs to be re-examined. The old-age consumption of rural residents can no longer count on a small amount of cultivated land with contracting rights for farmers and the old-age security must be built on the public pension security system. The government should focus on strengthening the capacity of rural old-age services.