In order to study the effect of ferronickel slag powder on the permeability of cement soil， the permeability of ferronickel slag powder cement soil in clean water environment and seawater environment was studied by a pressurized infiltration device， and the microscopic analysis was carried out by mercury intrusion test and scanning electron microscope-energy spectrum analysis test. The results show that the addition of ferronickel slag powder can improve the impermeability of cement soil in both clean water environment and seawater environment， especially in seawater environment. When the content of ferronickel slag powder is more than 20%， the effect of increasing the content of ferronickel slag powder on the impermeability of soil-cement gradually decreases. The ferronickel slag powder can play a micro-aggregate effect and activity effect， reduce the maximum pore size and total porosity of cement soil， and enhance its resistance to seawater erosion. The ferronickel slag powder can make the microscopic structure of cement soil more compact， and promote the formation of hydration products with low calcium-silicon ratio， and enhance its impermeability.