The sign-alternating electric field in rectangular impulses has been used to eliminate linear electrophoresis and to study nonlinear electrophoresis in water at strong fields, We found out that the particle drift velocity could change its direction with the growth of the strength of the field. This new phenomenon has obtained its explanation in the framework of the Debye-Huckel theory of strong electrolyte where we consider a particle as a "heavy" effective ion and take into account all relevant nonlinear effects, grounding ourselves on basic physics, With the help of our theory we have succeeded in good fitting of our experimental data on black oil and Al2O3 particles in distilled water using reasonable values for the basic parameters such as the Debye screening length.