The effects of size and volume fraction of poplar fiber on flowability, mechanical properties (compressive strength, flexural strength), and auto-shrinkage of mortar were studied by using control variable method. The results indicate that the volume fraction of poplar fiber has a more significant effect on flowability of mortar than its size. When the volume fraction of poplar fiber increases from 1.0% to 3.0%, the flowability of mortar decreases by 49.56%. Increasing the size and volume fraction of poplar fiber reduce the flexural strength of mortar, while increasing the volume fraction of poplar fiber is conducive to improving the compressive strength of mortar. The volume fraction of poplar fiber is more significant than its size on the effect of reducing shrinkage of mortar, and the addition of 3.0% poplar fiber of 0.30~1.25 mm reduces the auto-shrinkage of mortar by 45.6% at 72 h.