Abstract: Biochar is commonly used as an amendment to improve the properties of Vertisols with very sticky, easy sticking together, and poor tillage. Previous studies have focused on the effect of biochar dosage and type on the improvement of Vertisol. This study aims to clarify the effect of biochar particle size and dosage on the strength of Vertisol. Two typical Vertisols (silty clay loam and silty clay) were also selected. The simulation experiments were then carried out on the biochar with two particle sizes (1-2 mm and < 0.15 mm) and three dosages (1%, 3% and 5%). A systematic investigation was made to determine the tensile strength, cone penetration depth, and undrained shear strength of Vertisols. The results show that the tensile strength of silty clay loam decreased by 19.5% and 63.8% in the 3% and 5% coarse biochar treatments, respectively, compared with the control. The tensile strength of the silty clay decreased by 39.7%, 71.2%, and 166.2% in the 1%, 3%, and 5% coarse-grained biochar treatments, respectively, compared with the control. Thus, the coarse-grained biochar treatment significantly reduced the tensile strength of Vertisols. The higher the biochar dosage was, the lower the tensile strength was. Biochar was lesser reduced the tensile strength of silty clay loam than that of silty clay. In contrast, the fine-grained biochar treatment failed to reduce the tensile strength of Vertisols. Specifically, the tensile strength of silty clay loam and silty clay after 1% fine-grained biochar treatment was 16.0% and 21.2% higher than those of the control, respectively. Biochar treatment increased the slope of the regression equation for the cone penetration depth and water content. Particularly, there was a greater increase in the silt clay than that in the silty clay loam. The slope of silty clay loam increased by 1.3% to 27.4%, while the slope of silt clay increased by 35.6% to 53.9% after biochar treatment, compared with the control. The relationship between undrained shear strength and water content was better fitted by logarithmic function (R² > 0.941, RMSE < 0.210 kPa) under different biochar treatments. In silty clay loam, 1% coarse-grained and 1% and 3% fine-grained biochar treatments shared little effect on the cone penetration depth and undrained water shear strength. But 3% and 5% coarse-grained and 5% fine-grained biochar treatments reduced the cone penetration depth, where there was an increase in the undrained shear strength. In the silty clay, 3% and 5% coarse-grained biochar generally reduced the cone penetration depth, whereas there was an increase in the undrained shear strength. Once the water content was higher than 0.45 g/g, the fine-grained biochar treatment increased the cone penetration depth, but there was a decrease in the undrained shear strength. As such, the effect of biochar application on the strength of Vertisols was closely related to the biochar particle size and dosage, soil properties, and water content. The reason was that the biochar adjusted the cohesion, internal friction angle, and suction stress among soil particles.