Abstract: The loess contains a large amount of Na⁺, Ca²⁺, Cl^-, SO₄^2-, etc. When the water enters loess, it dissolves these ions in the water and carries mineral ions away. It affects the structure and strength of loess. In order to study the influence of mineral ions' concentration on the strength of loess, we used the TFB-1 type triaxial apparatus of stress and strain controlled for unsaturated soil and carried out the consolidation and undrained(CU)test with remolded sample from Heifangtai in Gansu Province. We did CU test with the samples of loess reconstituted with different concentration's solution of sodium sulfate under the confining pressures of 100 kPa, 200 kPa and 300 kPa, respectively. We discussed the effect of sodium sulfate's concentration on strength of loess. The test result shows that under the same confining pressure, the peak shear strength and residual shear strength of loess decrease first and then increase with the increase of sodium sulfate's concentration. Under the same dry density, with the increase of sodium sulfate's concentration, the total internal friction angle, effective internal friction angle and total cohesion of loess decreases first and then increases, while the effective cohesion of loess increases monotonously. The increase of sodium sulfate's concentration reduces thickness of clay's double electric layer, which enhances the force between clay particles. So the aggregate of clay particles increases, which affects the strength of the loess.