The emergence of chiefdom-like societies in the middle-lower Yellow River valley during the late Longshan period(4. 5~4. OkaB. P.) marks a very important stages in the evolution of ancient Chinese civilizations,attracting scholars in various academic disciplines. However,up to date,no hypothesis has ever given a satisfactory explanation on the mechanisms behind such an important social development. One reason is that most scholars neglected the possible role of climatic change in the evolution of ranked societies. However, recently, increasing research has substantiated the fact that climatic change may have played very role in the evolution of ancient civilizations. In this paper, previous hypotheses on the dynamic mechanism behind the emergence of chiefdoms are reviewed and rejected, and an alternative model was introduced. Firstly, we assembled the archaeological evidence that pointed to prevalence of warfare during the Longshan period,which supports the popular hypothesis that war is the main mechanism behind the social evolution. Secondly, we reconstructed the climate background of the Longshan Period during which the chiefdoms emerged in the middle-lower Yellow River valley,and found that a climatic anomaly occurred during 4. 5~4. OkaB. P. The onset of such climatic cooling coincided closely in timing with the emergence of chiefdom-like societies in the middle-lower Yellow River valley,indicating a possible relationship between them. Based on analysis of the demographic and social evolution theories, we developed alternative model for the emergence of chiefdom-like societies in the middle-lower Yellow River valley. Our model invoked abrupt climate change as independent variable,while seeing population growth as a necessary condition rather than sufficient one and incorporating social circumscription induced mainly by population growth. This model can be summarized as:in a circumscribed environment, abrupt climate change around 4. 5~4. OkaB. P. acted on certain degree of population density level, resulting in population-resource imbalance. Such resource stress further triggered outbreaks of warfare which may eventually lead to the evolution of chiefdom-like societies in the middle-lower Yellow River valley. Our model has some advantages over others in that it could not only explain the mechanism and process behind the emergence of chiefdoms but also answer very well the questions that why the chiefdom-like societies occurred synchronously during 4. 5-4. OkaB. P. across the vast areas in the middle and lower Yellow River valley.