To improve the utilization of phragmites in biorefineries, the strengthening effect of byproduct-organic acids on the subcritical liquid hot water (SLHW) pretreatment was explored. The byproduct-organic acids were accumulated during the direct recycling of pretreated liquid, which could reduce water consumption. The byproduct-organic acids, including acetic acid and lactic acid, were used to strengthen the SLHW pretreatment of raw phragmites at different temperatures (160-200 degrees C) and times (20-90 min) in the strengthening process. During the SLHW pretreatment, the high yield of C-S sugars was 60.66% at the severity of 4.85 (200 degrees C, 75 min). However, the maximum C-S sugars yield in the strengthening process was 85.01% at 180 degrees C, for SO min, and with 2.106 wt % byproduct-organic acids, which were obtained by the direct recycling of pretreated liquid for 3 times at 160 degrees C and for 30 min. Therefore, the full utilization of byproduct-organic acids could provide another economically feasible way to strengthen the SLHW pretreatment.