Hydrothermal processing technology, a process for treatment of wet biomass for biofuel production, produces a byproduct wastewater (aqueous phase, AP) during bio-oil production, which is becoming an impeding factor for the industrialization of the hydrothermal process. Chemical compositions and wastewater properties of AP produced from hydrothermal processing wet biomass provides very important information for its utilization. The present manuscript reviewed the recent development on production and chemical compositions and wastewater properties of AP derived from the hydrothermal processing of wet biomass. The relationships between AP properties and biomass compositions and processing conditions were discussed. The possible reaction pathways account for the resultant AP was also discussed. The present review would be useful to give comprehensive information on AP and contribute to the investigation on effective utilization of AP.