The replacement of river sand in the mortar by using iron ore tailings (IOT) can not only turn waste tailings into wealth but also protect the environment. In order to further improve the physico-mechanical properties and cracking resistance of cement mortar with 50% IOT, a series of experiments were conducted by adding different contents of basalt fiber (BF) and polyacrylonitrile fiber (PANF). The consistency decreases significantly with increasing the BF and PANF contents. A proper content of fibers can be evenly distributed in the mortar and thus reduce the porosity. However, excessive fibers will aggregate and increase the porosity. In addition, it has been validated that fibers can significantly improve the strength and prevent the brittle cracking. With increasing fibers, the density, ultrasonic velocity, and mechanical strength of hardened fiber-reinforced mortar first increase and then decrease. The best adding contents of BF and PANF are 1.0% and 0.08%, respectively. The rigid BF (length=6 mm) can significantly increase the uniaxial compressive strength (UCS) by 32.57% due to its good shear-resistance ability, however, the flexible PANF with the length of 12 mm is better to improve the split-tensile strength (STS) and flexural strength (FS) by 22.63% and 17.43% due to the formation of three-dimensional network structure in the specimens. In addition, the scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests show that excessive fibers will agglomerate and produce a certain amount of air, while a proper adding content of fibers provides more attachment points for hydration products to promote the hydration reaction. It is suggested to adopt the best adding content and choose proper fiber types according to the loading form of construction structure with tailing cement mortar in the fields.