In arid and semi-arid regions

salinization caused by high evaporation rate of soil will pose a potential threat to the durability of engineering structures. Therefore

accurate prediction of soil pores relative humidity is of great significance for guiding salt control of engineering structures. Based on the conclusion that the chemical potential of soil pores water is always in equilibrium with pores vapors

a relative humidity expression of unsaturated saline soil pores is deduced according to the principle of chemical equilibrium. By comparing the relative humidity of calculated and the measured solutions

the results can be obtained that the calculated values of relative humidity at different temperatures and concentrations are in good agreement with the experimental values

and are superior to that by the R&S model; the coincidence effect is more obvious especially when the concentration of solute is nearly saturated. Meanwhile

it is found that the numerical results are in good agreement with the experimental results by substituting the proposed relative humidity expression into the existing numerical model without changing the calculation parameters

especially in the prediction of evaporation rate and volume water content. In conclusion

the new formula for calculating pore relative humidity of unsaturated saline soil is effective in the process of water-salt migration calculation.