Anxiety is a frequently perceived emotional state in today’s society. Research on horticultural therapy in natural environments has shown that horticultural therapy can effectively relieve anxiety. Through a systematic review of research on horticultural therapy, this article first presents the factors about the intervention effect of horticultural therapy on anxiety. Then, by elaborating on evolutionary perspectives such as biophilia hypothesis, attention restoration theory, stress recovery theory, and neurophysiological mechanisms, as well as theoretical hypotheses from physiological and psychological perspectives, this article attempts to reveal the mechanism by which horticultural therapy intervenes in anxiety. Related research of horticultural therapy should further explore the differences in the effectiveness of horticultural therapy in anxiety intervention, attempt to explore the sustained effects of horticultural therapy, and focus on the design of intervention models for different populations.