It is a phase I/II clinical study to evaluate the safety, tolerability and preliminary efficacy of VUM02 Injection in patients with acute graft-versus-host disease (aGvHD) who have failed systemic steroid therapy. VUM02 Injection (human umbilical cord-derived mesenchymal stromal /stem cells, hUC-MSC) is an off-the-shelf allogeneic cell therapy product comprising culture-expanded mesenchymal stromal /stem cells derived from the human umbilical cord tissue. The product is cryopreserved with the cell concentration of 5 x 10\^6 cells/mL. Patients with grade II to IV aGvHD who have failed systemic steroid therapy (i.e. patients with steroid-refractory aGvHD (SR-aGvHD)), will be recruited into this study. This study consists of two phases, a dose-escalation phase (phase I) and a dose-expansion phase (phase II).