Abstract: The influences of positive pressure, relative sliding rate and lubrication condition on friction coefficient between sealing rubber of downhole bop and casing pipe were studied by friction test. The results show that the friction coefficient increased with the sliding rate increasing at the same positive pressure without lubricant or water lubrication, but decreased in oil lubrication because the growth of sliding rate made contribution to the formation of lubricating film. At the same pressure and relative rate, the average friction coefficient in oil lubrication was the smallest, followed by that in water lubrication and the greatest was given without lubricant. The effect of positive pressure had no regularity.