In order to evaluate the improvement effect of fiber on the brittle failure of cement-treated subgrade soil, a series of triaxial unconsolidated undrained (UU) tests were carried out on samples of polypropylene fiber-cement-treated subgrade soil (PCS) with polypropylene fiber mass content of 0‰, 2‰, 4‰, 6‰, and 10‰. The results showed that, (1) the deviatoric stress-axial strain curve of PCS samples were all strain-softening curves. (2) For the same fiber mass content, the peak stress, residual stress, and strain at peak stress of PCS samples gradually increases with the increase in the confining pressure, while their brittleness index gradually decreases. (3) With the increase in confining pressure, compared with that of the 0‰ PCS sample, the increase in peak stress, residual stress, and strain at peak stress of 6‰ PCS sample were in the ranges of 24%–29%, 87%–110%, and 85%–120%, respectively. The decrease in the brittleness index and failure angle was 52%–79% and 16%, while the cohesion and internal friction angle increased by 25.9% and 7.4%, respectively. The results of this study indicate that it is feasible to modify cement subgrade soil with an appropriate amount of polypropylene fiber to mitigate its brittle failure.