Abstract: In order to master the mine strata pressure behavior law of ultra long fully-mechanized coal mining face and the causing mechanism, a method with a sit e measurement and a theoretical analysis was applied to analyze the mine strata pressure behavior law of the 450 m ultra long fully-mechanized coal mining face of Hal agou Mine. The results showed that there were large and small periodical roof weighting phenomena existed in the 450 m ultra long fully-mechanized coal mining face, a large periodical roof weighting would occurr at an interval of 8 m after tow small periodical roof weightings and during the large periodical roof weighting period, the te ndency pressure distribution of the coal mining face would have three peaks with W features. The large and small periodical roof weighing features occurred in the coal mining face were caused by the joint role of the secondary key stratum and main key stratum. The broken of the main key stratum could cause the secondary key stratu m to be broken in advance and thus the roof weighting distance and the roof weighting intensity of the coal mining face would be alternatively varied in size. During the periodical roof weighting period, the basic roof would be broken individually in the pressure peak region. With the coal mining face pushing forward, each broken rock bl ock would have rotary subsidence along the coal mining face tendency and the working resistance of the powered supports in the near region would be increased.