Abstract: The characteristics and causes of the climate change over China during Medieval Warm Period(MWP)and Present Warm Period(PWP)are analyzed in this paper, based on the climatic simulation results over the past 1500 years using the Co mmunity Earth System Model(CESM). Five experiments are compared, including the solar forcing run(TSI), volcanic forcing run(VOL), well-mixed greenhouse gas forcing run(GHGs), land use/Land cover forcing run(LUCC)and 4-forcing run(MASK). The experiments are carried out from 1A.D. to 2000A.D. based on a 2400 years' control run under the A.D. 1850's initial forcing, except the VOL run, which only span from 500A.D. to 2000A.D., limited by the reconstruction of the volcanic forcing. Year 500~2000 is used in this study. We compared the climatology of the temperature and the precipitation derived from the MASK run to the reanalysis data, reconstructions and those derived from the other model simulation data, in order to do the model validation. The characteristics of the simulated temperature and the precipitation by the CESM show similarity to the reanalysis data and the reconstructions. There are three typical climate periods-MWP, Little Ice Age(LIA)and PWP-simulated by the MASK run, which are consistent with the reconstructions. In this study, the two 100 years' persistent warming periods during the MWP(1001~1100A.D.)and the PWP(1901~2000A.D.)are compared. The method of the empirical orthogonal function(EOF)is used in this study to find out the characteristics of the temperature and the precipitation during the two warm periods. The results show that, the spatial patterns of temperature variability over China are different during the two typical warm periods, while the spatial patterns of the precipitation variability are similar. The abrupt change of the temperature during the PWP, caused by the change of the greenhouse gas concentration, is absent during the MWP. The main cause of the temperature change is the greenhouse gas concentration during the PWP, and the solar radiation during the MWP. The solar radiation is the direct cause to the multi-decadal variation of the temperature. The precipitation change during the two warm periods both illustrate the out-of-phase pattern between eastern and western, and between southern and northern parts of China. The factors affecting the spatial and temporal changes of the precipitation are complicated and need further investigation. The approximately five year circle of the precipitation variation is affected by the forcings indirectly, which is also hungered for more studies. It is more important to point out that the preliminary results here cannot assess the quantitative contributions of the four external factors to the climate over China. More works are needed for understanding the relative contributions of each factor to the changes of the temperature and the precipitation during the two warm periods.