Horizontal diffussion cells were used to observe and measure the diffussion processes of the water soluable salt cations in engineering loess.The results obtained showed that the diffussion characteristics of every cation changed with its variety.The diffussion coefficient of Ca²⁺was the highest,and that of Mg²⁺, Na⁺ and K⁺ dropped in turn.The mean values of them determined in the two models were 2.692 8×10^-6,2.312 7×10^-6,1.6370×10^-6 and 0.4504×10^-6cm²/s respectively.The significant increase of the exchangeable K⁺, Na⁺ and Mg²⁺,and the decrease of the exchangeable Ca²⁺in soil near the source solution indicated that the formers were absorpted and the later was desorpted by the soil collision.