We study the intervalley scattering in defected graphene by low temperature transport measurements.The scattering rate is strongly suppressed when defects are charged.The finding highlights 'screening' of the short-range part of a potential by the long-range part.Experiments on calcium adsorbed graphene confirm the role of a long-range Coulomb potential.This effect does not rely on the particular band structure and is applicable to other multivalley systems.Our result provides a means to electrically control valley relaxation and has important implications in valley dynamics in valleytronic materials.