In order to study the mechanical behavior of recycled fine aggregate mortar, 11 kinds of different recycled fine aggregate replacement rate (ranged from 0% to 100%, level differential 10%) of cement mortar test specimens are designed. The failure pattern and the mechanical performance indexes of the cement mortar under different recycled fine aggregate replacement rate are gained by test. And the different physical indexes of natural fine aggregate and recycled fine aggregate are measured in detail. The experiment findings indicate that because the water absorption rate of the mortar with high porosity is higher, and there are mass microcracks in recycled fine aggregate interior due to damage accumulation, the lower apparent density, the higher water absorption rate and the quicker water absorption speed of recycled fine aggregate are caused. So the fluidity of recycled fine aggregate mortar is fine, but the water retention is bad, the compressive strength is lower than the natural fine aggregate mortar about 50%. But the replacement rate has little effect on the mortar strength.