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The electrochemical property, stress corrosion cracking（SCC）susceptibility of X70 pipeline steel before and after annealing was investigated by potentiodynamic polarization techniques, slow strain rate test（SSRT）and optical metallographic microstructure observation, to detect the effect of the microstructure on the electrochemical reactions and how this effect works on the SCC behaviour in an acidic soil environment. It was demonstrated that the coarse grain can intensify the cathodic electrochemical reactions because that the grain surface of the steel is more active compared to the grain boundary. This effect introduces more hydrogen into the metal matrix which leads to a higher SCC susceptibility of the annealed X70 steel rather than the as received steel.