Abstract: With the simulation experiments on diagenetic transformation of mont-morillonite,the primary migration of hydrocarbon is studied. Montmorillo-nite clay specimens are selected from various areas and the simulating experiments are carried out with certain conditions of temperature, pressure and water medium.Also the X-ray diffraction and thermal analyses are made on the clay mineral speciments from some wells in North Jiangsu, Depending on the experiments and analogy analysis, the obvious regularity of diagenetic transformation from shallow to deep is discovered, The main mineral compositions in these wells are expansible montmorillonite at depths of about 2000 m, mixed layer clay of non-expansible illite and montmorillonite at depths of 2500-3800m, and non-expansible illite at depths deeper than 3800 m, In addition, clay of montmorillonite-illite mixed layers are found within the related oil and gas beds, which supports the viewpoint that pore interstitial pressure is created by dehydration during the transformation from montmorillonite to illite.It provides a favourable environment for expulsion of liquid hydrocarbons.