Based on the triple-shear unified strength criterion and shear strength of unsaturated soil in terms of two state stress variables, a triple-shear unified solution of shear strength for unsaturated soil is obtained with considering the effects of the intermediate principal stress, strength disparity of materials and strength criterion. A formulation of Terzaghi’s ultimate bearing capacity for unsaturated soil foundation is developed, and validated through the feasibility analysis and example demonstration. In addition, the influences of various factors, including intermediate principal stress effect, strength criterion, matric suction, effective internal frictional angle and effective cohesion, are discussed on the ultimate bearing capacity of unsaturated soil foundation. The results show that the intermediate principal stress and strength criterion have significant impact on the ultimate bearing capacity, namely, the more remarkable the intermediate principal stress effect, the higher the ultimate bearing capacity is. It is also shown that the consideration of intermediate principal stress can fully mobilize the soil strength and the strength criterion plays an important role in predicting the ultimate bearing capacity. The matric suction has dual effects on the Terzaghi ultimate bearing capacity of strip foundation. In low matric suction region, the ultimate bearing capacity increases linearly with the increase of matric suction and attains its maximum when the matric suction reaches the air-entry value. In high matric suction region, the ultimate bearing capacity decreases gradually and finally maintains constant. Moreover, the Terzaghi ultimate bearing capacity increases dramatically as the effective internal frictional angle and effective cohesion increase.