The influences of sepiolite fiber volume fraction and length on mechanical properties and drying shrinkage of artificial sand mortar with high stone powder content were investigated. A mercury porosimeter and a scanning electron microscope were employed to analyse the characteristic variation of mortar internal pore structure and microscopic morphology. The results indicate that an appropriate quantity of short sepiolite fiber significantly improves the compressive strength, flexural strength and drying shrinkage resistance of mortar. Compared to the blank group, the 28 d compressive strength and flexural strength of specimens with 1.5% volume fraction of 1 mm length fiber increase by 98.9% and 36.2%, respectively. Besides, the 28 d natural drying shrinkage value with 2.0% volume fraction of 1 mm length fiber reduces by 72.1%. A large number of needle-shaped ettringite and flaky calcium hydroxide crystals are observed inside the sepiolite fiber specimen, which effectively improve the compactness of mortar hardening system. The total porosity of mortar is inversely proportional to the fiber volume fraction.