To investigate variation characteristics of interlayer water bound up with organic matter in smectite, organo-clay complexes extracted from grinded source rock samples were determined using thermo-XRD, DTA and PY-GC. The d001 diffraction peak of organo-clay complexes is postponed from 250 to 550°C before reaching 1.00 nm and accompanied by exothermal peaks on DTA and organic matter with abundant C20-C30 carbon detected by PY-GC, which is different from single smectite and indicates the existence of organic matter in the interlayer of smectite. Water desorption characteristics of organo-clay complexes are in consistent with smectite at 100 and 600°C, but different from smectite at 550°C with an additional dehydration peak and a remaining d001 diffraction peak, suggesting the water removed at 550°C is interlayer adsorption water rather than constituent water of clay minerals. Comparing the dehydration order and water loss, we conclude that part of interlayer water of smectite may act as the “bridge” that binds organic matter and smectite, which results in water-expelled lag beyond 250°C and may provide a good medium for hydrocarbon migration and oil pool formation. To investigate variation characteristics of interlayer water bound up with organic matter in smectite, organo-clay complexes extracted from grinded source rock samples were determined using thermo-XRD, DTA and PY-GC. The d001 diffraction peak of organo-clay complexes is postponed from 250 to 550 ° C before reaching 1.00 nm and accompanied by exothermal peaks on DTA and organic matter with abundant C20-C30 carbon detected by PY-GC, which is different from single smectite and indicates the existence of organic matter in the interlayer of smectite. Water desorption characteristics of organo-clay complexes are in consistent with smectite at 100 and 600 ° C, but different from smectite at 550 ° C with an additional dehydration peak and a remaining d001 diffraction peak, suggesting the water removed at 550 ° C is interlayer adsorption water rather than constituent water of clay minerals. Comparing the dehydration order and water loss, we conclude that part of interlayer water of smectite may act as the “bridge ” that binds organic matter and smectite, which results in water-expelled lag beyond 250 ° C and may provide a good medium for hydrocarbon migration and oil pool formation.