One of the leading causes of deaths in Humans is cancer. In recent years, nanomedicine has attracted significant attention as a therapeutic strategy, i.e., drag delivery system (DDS) for delivering medicine encapsulated in nanoparticles to target tumors. However, the prediction of distributions of injected nanoparticles throughout a hierarchical vascular network remains quite challenging because their transport is strongly influenced by coupled dynamical and chemical interactions with red blood cells (RBCs), vessel walls, and plasma. To solve the issue, multi-scale and multi-physics simulations reproducing the dynamics of blood flow, RBCs and nanoparticles and their coupling in silico play a critical role. Dissipative particle dynamics (DPD) is a promising numerical approach because it is a Lagrangian method that allows to handle highly deformable, complex geometries such as RBCs and also to incorporate chemical effects. One bottleneck is that most existing DPD simulations rely on exploiting periodic boundary conditions, which makes it difficult to explicitly impose inlet boundary conditions such as hematocrit and nanoparticle density. One promising solution is the inflow/outflow boundary condition proposed by Lykov et al.(2015), where the DPD particles are copied from another simulation in a subdomain to provide the inlet condition of the main computational domain, and DPD particles near the outlet are properly deleted for imposing a desired outlet condition. Nevertheless, a code implementing the above algorithm is not currently available to the public, and can run on the CPUs. Considering future applications to biological and medical problems, GPU acceleration is strongly desired. In this work, we implement the inflow/outflow boundary condition in USER-MESO 2.5 (Xia et al., 2020), an open-source, CUDA-based GPU-accelerated DPD solver. We validate the implementation using benchmark test cases and demonstrate its potentials to applications to biology, medicine, etc. Eventually, we plan to release USER-MESO3.0, including codes and tools for various blood flow simulations with the new inlet and outlet conditions.