Human-centric spatio-temporal video grounding (HC-STVG) is a challenging task that aims to localize the spatio-temporal tube of the target person in a video based on a natural language description. In this report, we present our approach for this challenging HC-STVG task. Specifically, based on the TubeDETR framework, we propose two cascaded decoders to decouple spatial and temporal grounding, which allows the model to capture respective favorable features for these two grounding subtasks. We also devise a multi-stage inference strategy to reason about the target in a coarse-to-fine manner and thereby produce more precise grounding results for the target. To further improve accuracy, we propose a model ensemble strategy that incorporates the results of models with better performance in spatial or temporal grounding. We validated the effectiveness of our proposed method on the HC-STVG 2.0 dataset and won second place in the HC-STVG track of the 4th Person in Context (PIC) workshop at ACM MM 2022.