Hydrothermal carbonization (HTC) of polyvinyl chloride (PVC) to produce high-value-added hydrochars as a solid fuel alternative can play a pivotal role in carbon sequestration and energy supply. However, there is unclear for the interaction mechanism and combustion performance of co-hydrochars derived from PVC waste and biomass. In this study, waste PVC mats (PVCM) mixed with wheat straw (WS) were as feedstocks to prepare the co-hydrochars by the acid-catalyzed co-HTC at different hydrothermal temperatures and mass ratios. The presence of WS enhanced the PVCM substitution and prevented the condensation of the PVCM-derived hydrochar aggregation. In turn, PVCM played a stimulative role in the WS decomposition. Meanwhile, the interactions were improved sufficiently in the acidic mediums. The fuel properties and combustion behavior of co-hydrochars were elevated by valid interactions between PVCM and WS. The co-hydrochar obtained using co-HTC at a mass ratio of 1:1 under 240 °C (HC-240–1:1) exhibited the optimal higher heating value (32.72 MJ/kg), splendid dechlorination efficiency (97.41%) as well as worthy fuel ratio (0.89). The combustion process of HC-240–1:1 was more facile and consistent as a result of the elevated ignition and burnout temperatures along with reduced activation energies. Furthermore, the liquid phase products were recycled to assess the reusability. The process water expedited the process of co-HTC, which was confirmed by the improved HHV (32.72 MJ/kg to 39.71 MJ/kg) and mass yield (30.73% to 32.44%). The strategy of PVC waste mixed with biomass through the acid-catalyzed co-HTC was considered a promising way to produce environmentally friendly solid fuels, providing some theoretical basis for the commercialization of co-HTC.