The invention discloses a determination method for compression strength ratio of low-quality recycled fine aggregate mixed mortar. The determination method comprises the steps of firstly determining saturated-surface-dry water adsorption of the low-quality recycled fine aggregate and common sand; drying to a constant weight; weighing dried common sand and cement; mixing the dried common sand with the cement; adding water with stirring to obtain mortar until the fluidity of the mortar is (130+/-5) mm; making a benchmark mortar test specimen from the mortar; testing after 28 days of standard curing; recording a strength being f[0] MPa; then mixing the low-quality recycled fine aggregate and the common sand to obtain mixed mortar; weighing the mixed mortar and the cement; mixing the mixed mortar with the cement; adding water in the mixture to prepare a contrast mortar specimen; testing after 28 days of standard curing; and recording a strength being f MPa. The mortar compression strength ratio (delta) after 28 days of standard curing is equal to f/f[0]. The determination method takes additional water consumption while determining the mixed mortar strength ratio, and can actually reflect the performances of the mixed recycled fine aggregate.