Since　the　traditional　penetration　testing　of　cement　soil　is　time-consuming,　this　paper　introduces　a　new　electrochemical　testing　method　to　study　the　effect　of　ferronickel　slag　powder　on　improving　the　impermeability　of　cement　soil　in　the　corrosive　seawater　environment.　To　this　end,　the　mercury　intrusion　test　was　employed　to　analyze　the　change　in　the　pore　characteristics　of　cement　soil,　and　the　variation　in　the　impermeability　of　cement　soil　was　further　explained　from　a　microscopic　point　of　view.　The　test　results　show　that　using　the　electrochemical　method　to　test　the　impermeability　of　cement　soil　dramatically　improves　the　efficiency　of　testing.　The　impermeability　of　cement　soil　gradually　rises　with　an　increase　in　its　ferronickel　slag　powder　content.　Moreover,　the　optimal　mass　replacement　ratio　of　the　ferronickel　slag　powder　and　mineral　powder　composite　to　cement　is　40%　in　the　testing　range　of　0-40%.　The　mercury　intrusion　test　results　also　confirm　that　the　addition　of　ferronickel　slag　powder　can　decrease　the　most　probable　pore　size　and　total　porosity　of　cement　soil;　further,　from　a　microscopic　perspective,　the　ferronickel　slag　powder　can　help　reduce　the　corrosion　of　cement　soil　by　seawater.　The　above　test　conclusions　are　consistent　with　the　results　of　the　traditional　permeability　test.　Our　findings　not　only　imply　that　ferronickel　slag　powder　can　lessen　the　effect　of　seawater　on　the　corrosion　of　cement　soil　but　prove　that　the　electrochemical　test　is　a　reliable　and　straightforward　method　for　determining　the　impermeability　of　cement　soil.