Clay is a basic material of drilling fluid, dispersion behavior of clay-water suspension system after heat rolling aging has important influences on drilling fluid properties,especially high density drilling fluid,such as rheological property,fluid losing and wall building property,and sedimentation stability.Influencing Factors of particle size distribution of clay-water suspension system was studied by laser particle size analyzer including clay kinds,inorganic salt and sodium hydroxide.The results show that particle size distribution of clay-water suspension system such as kaolin,sepiolite and attapulgite are stable,however,particle size of sepiolite ia bigger than kaolin and attapulgite.With the increasing of aging temperature,particle size distribution of bentonite-water suspension system follow the law:below 90℃,hydration dispersion plays a dominant role,dispersity of bentonite increase;90℃-180℃,hydration dispersion and dehydration coalescence keep relative balance,dispersity of bentonite is stable;180℃-200℃, dehydration coalescence plays a dominant role,dispersity of bentonite decrease;addition of sodium hydroxide increased only dispersity of bentonite,dispersity difference disappearedmore than 150℃.With the increasing of aging temperature,particle size of bentonite-inorganicwater suspension system decreased,dispersity of bentonite increase and keep stable more than 120℃.Hydration dispersion and dehydration coalescence both go against the effect of drilling fluid,we can decrease or eliminate this disadvantageous effect by decreasing addition of bentonite,or using clay such as kaolin and attapulgite,or adding inorganic salts.