XplainAct is a visual analysis framework that supports simulating, explaining, and inferring the effects of interventions at the individual level. Unlike traditional causal inference methods that focus primarily on group-level effects, XplainAct supports individual-level analysis, considering that the effects of interventions can vary significantly across subgroups in highly heterogeneous systems. We demonstrate the effectiveness of XplainAct through two case studies: drug-related deaths and presidential election voting patterns.