Abstract: The dispersivity of dam soil samples in Nanping Reservoir is identified by means of double-hydrometer tests, pore water salt tests, pinhole tests, crumb tests and exchangeable sodium percent tests. Furthermore, the dispersive mechanism and reasons are also studied in combination with the analysis of physicochemical properties and mineral components. The results show that K16 and K19 are nondispersive, and K30 and Z10 are dispersive. Rich montmorillonoid and sodium-ion are the main reasons to cause the dispersivity of soil under the alkalic environment, and another key reason is the granulometric composition of soil. The cementation of clay, especially colloidal particle, plays an important role in the structural stability of soil samples. Much calcium ion and high organic matter can restrain the dispersivity of soil, and adding lime can improve it effectively.