Cathodic　protection　is　usually　used　to　control　the　corrosion　in　service　pipeline.　The　optimum　cathodic　protection　potential　(OCPP)　becomes　a　vital　part　in　order　to　get　the　best　protective　effect　in　the　whole　research　of　cathodic　protection.　Up　to　now,　depending　on　experience,　the　OCPP　is　often　defined　between　200mV　to　300mV　more　electronegative　than　the　free　corrosion　potential　under　usual　circumstances　(neutral　aqueous　solution　or　soil).　However,　the　phenomena　of　over-protection　and　under-protection　usually　occur　in　service　pipeline　with　the　complexity　of　soil　environment.　The　cathodic　protection　parameters　of　X100　pipeline　steel　in　Dagang,　Daqing　and　Lasa　simulated　soil　solutions　have　been　investigated　by　using　the　polarization　test　and　electrochemical　impedance　spectroscopy　(EIS).　The　value　of　R-t　(the　charge　transfer　resistance)　by　analyzing　EIS　curves　showed　that　the　polarization　resistance　of　the　specimen　had　a　maximum　value　with　negative　shift　of　the　polarization　potential.　If　the　value　was　negative　to　1000　Omega　center　dot　cm(2),　the　hydrogen　evolution　reaction　would　occur.　The　potential　of　hydrogen　evolution　in　the　three　simulated　soil　solutions　from　low　to　high　was　Dagang,　Daqing　and　Lasa.　This　research　could　offer　theoretical　guideline　and　technique　support　for　the　cathodic　protection　of　pipeline　steel.