For the quaternary salt-water systems

such as the homo-ion system K+//Cl-

SO42-

NO3--H2O and interaction system Na+

K+//SO42-

NO3--H2O

as well as their six ternary subsystems

the freezing points

eutectic points and the crystallization regularities of salts were studied by cooling method. According to experimental data

the eutectic point temperature-liquid composition diagrams for these systems were plotted. The results were as follows: (1) The crystallization regularity of salts and the eutectic point temperatures can be obtained by measuring the time-temperature curves for multicomponent salt-water systems during cooling process; (2) In these systems above

3K2SO4·Na2SO4 existing under normal temperature did not appear at the eutectic point temperature. As a result

the phase relationships in these systems were simplified; (3) The prism can be used to describe the relationships among the phases

freezing points or eutectic points and the compositions of liquids for the ternary systems

the quaternary homo-ion systems and interaction systems concisely and visually; (4) In the eutectic point temperature-liquid composition diagrams

for ternary systems without double-salts

there were two eutectic curves with single salt and ice

one eutectic point with two salts and ice

and one ice crystallizing region. For the quaternary homo-ion systems and interaction systems

there were three and four eutectic regions with single salt and ice

three and five eutectic curves with two salts and ice

and one and two eutectic points with three salts and ice

respectively.