Basic magnesium sulfate cement concrete (BMSC) has many advantages, such as quick setting, early strength, high tensile strength, resistance to corrosion, and so on. However, the failure of diagonal-tension of BMSC beams has not been studied. In order to explore the differences in mechanical performance between BMSC beams and Portland cement concrete (PC) beams, a comparative test of diagonal-tension failure of the BMSC and PC beams were conducted. The experimental results show that the BMSC beams have advantages in the cracking load and the ultimate bearing capacity compared with the PC beams under diagonal-tension failure. There are some differences in the failure mode between the BMSC and PC beams. The cracking load and the bearing capacity of the BMSC beams are 20% higher than that of the PC beams with the same reinforcement and the same concrete strength grade. So the calculation formulas for cracking load and bearing capacity of the PC beams have been modified. This paper has significance to the application of BMSC in civil engineering.