In this work, two-step sodium chlorite (SC)-acetic acid (AC) and AC-SC pretreatments were employed to produce xylooligosaccharides (XOS) and monosaccharides from poplar. The optimal conditions for SC pretreatment were 70 °C, 4.8% SC, and 3 h, which removed 70.3% of original lignin and retained more than 95% original glucan in poplar. The conditions for the AC pretreatment were 170 °C, 5% AC, and 30 min, giving a XOS yield of 37.3% from poplar. Both SC-AC and AC-SC pretreatments largely removed lignin, retained more than 82% glucan and produced XOS. In the hydrolysis of SC-AC- and AC-SC-pretreated poplar, glucose yields reached 99% and 92%, respectively, after the supplementation of 1.0 mg/mL Tween 80 with a cellulase loading of 10 FPU/g dry matter. More XOS and monosaccharides were produced from the AC-SC-pretreated poplar when compared with those from the SC-AC-pretreated poplar, and 40.0 g glucose, 4.0 g XOS, and 6.6 g xylose were produced from 100 g poplar. This work provides a relatively safe and cost-effective strategy for converting poplar into XOS and fermentable sugars.