Grain yield,as measured by grain output per unit harvested area,may be responsible for the improvement of gross grain output in China since 1978. Identifying major factors that account for such an increase and the fluctuation of grain yield and patterns of change may contribute to modification of food policy and food security. Starting from the rational-farmer hypothesis and technology-economy system analysis,we decomposed time-series(1978-2011)grain yield into different components using two-stage de-trending method. In addition,multi-scale fluctuation and major influencing factors of grain yield were also examined. We found that time-series grain yield could be decomposed into three parts,including a short-term fluctuation obtained by de-trending moving averages from time-series grain yield,long-term trend by curve fitting of moving averages and a medium-term fluctuation by subtracting the previous two. Short-term fluctuation was primarily determined by climatic factors;medium fluctuation by social-economic and policy cycles;and long-term fluctuation by technology and farming systems. Despite volatile time intervals,grain yield featured on overall long-term up-ward trend with little fluctuation. The degree of fluctuation as measured by maximum,minimum and range of fluctuation rates in the medium-term was larger than that over the short-term. Meanwhile,the degree of short-term fluctuation seemed to decline over the study period,and a similar trend was not observed for medium-term fluctuation. We conclude that medium-term or social-economic and policy cycles may be the largest contributor to grain yield fluctuation and this finding has significant scientific and policy implications for Chinas food policy.