Abstract: When grain is stored in the granary, it will be subjected to static and dynamic loads under the circumstances such as loading and unloading, earthquake and so on. And then these effects will act on the wall of the granary, affecting the stress of the wall. A new dynamic direct shear apparatus which consisted of shear box connecting components, horizontal thrust system and data acquisition system was used to provide a test platform for testing the mechanical properties of grain under static and dynamic loads. In order to investigate the dynamic mechanics of grain，the wheat direct shear experiments under static and dynamic loads were carried out. Meanwhile, the horizontal shear stress, shear displacement, vertical displacement and other relations in the dynamic test results were also analyzed. Using the new dynamic direct shear apparatus for grain, a series of tests were designed to study the dynamic characteristics of reciprocating shear considering three different porosities of 39%, 36%, 33% and four vertical pressures of 50, 100, 150, 200 kPa. These test results of strength and deformation were compared among the monotonic and post-reciprocating monotonic loading direct tests. The results showed that the stress-strain curves showed a softer curve and less dilatancy phenomenon with increasing vertical pressure during the monotonic direct shear test for the same porosity, thus a smaller dilatancy angle was also obtained. Under the vertical pressure of 50, 100, 150, 200 kPa, the corresponding dilatancy angles were 4.5°, 3.6°, 2.4°, 0.7°, While with the decreasing wheat porosity, the softening phenomenon became more obvious and the internal friction angle became larger. The internal friction angles of wheat with porosity of 39%, 36%, and 33% were 22.2°, 24.4°, 28.1°. What's more, the dilatancy phenomenon became more obvious with the decreasing porosity, inducing the increasing dilatancy angle. The corresponding dilatancy angles are 1.3°, 3.6°, 9.9°during the monotonic direct shear test. In the reciprocating direct shear test, with the decrease of porosity, the softening phenomenon of wheat was more obvious, and the dilatancy phenomenon was less evident. The maximum dilatancy are decrease with decreasing porosity, which corresponding with the maximum dilatancy deformation were 2.77, 1.74, 1.15 mm. These results of monotonic direct shear tests and post-reciprocating monotonic direct shear tests were compared. The results showed that the shear strength of wheat after reciprocating shear were significantly higher than those directly from the monotonic direct shear tests. The shear dilatation was also more obvious after reciprocating shear, corresponding with larger dilatancy deformation. The results showed that the strength and dilatancy characteristics of wheat were changed after the dynamic effect, which would affect the lateral pressure on the silo wall, which indicated that the lateral pressure coefficient of the silo wall need to be further improved when considering the dynamic effect. The results take into account the loading and unloading process, the static and dynamic characteristics of vibration, and the static characteristics after the dynamic effect. These results are necessarily provided for the engineers in the design and dynamic analysis of granary structure.