Abstract: The present paper expounds the salt index of fertilizers. The salt indexes in pure water and 3 soil true solutions of 12 fertilizers have been determined by electrical conductivity. The results show that:1. The salt indexes of different fertilizers vary from 8 (SSP) to 118(KC) based on salt index 100 of ammonium nitrate (AN).2. When indicting the differences of salt indexes between fertilizers, the actual weight of fertilizers is more suitable than nutrient weight.3. The trends of salt index differences between various fertilizers are similar in pure water or in soil true solutions, while the salt index of urea must be determined by soil true solution.4. The numerical value and their order of salt indexes between different fertilizers determined by eletrical conductivity or osmotic pressure are similar.The salt index of fertilizers, a very important parameter of affecting the soil water retaining capacity and supply of soil available water, can be used effectively to select the fertilizers with seed, work out the formula of compound or mixed fertilizers, determine the fertilization rate and its applying methods and so on.