Tree species diversity is widely assumed to correlate positively with structural complexity in forests, as differences in crown architecture among species enhance structural complexity through physical complementarity. While this complex-diversity relationship has been confirmed experimentally, it frequently weakens or reverses in natural forests. The mechanisms underlying this inconsistency remain unclear. We used the field measurements and drone-derived LiDAR data from the Tiantong 20-ha forest community in eastern China to calculate tree species diversity and forest structural complexity. To assess the mechanism driving the complex-diversity relationship, we calculated stand density and canopy height. We try to answer the following questions: (a) What is the relationship between structural complexity and species diversity in the forest plot? (b) How stand density or canopy height mediates the interplay between structural complexity and species diversity? (c) When controlling for stand density and canopy height effects, would a positive complexity-diversity relationship be revealed? We revealed that canopy height and stand density jointly mediate the observed negative complexity-diversity relationship: tall canopies reduced understory species diversity via shading, disproportionately excluding shade-intolerant species; while high stand density suppressed structural complexity, possibly by limiting vertical branching. When these mediating variables were statistically controlled for, a positive complexity-diversity relationship emerged. Our findings, therefore, resolve the apparent inconsistency by demonstrating that structural complexity and diversity inherently facilitate mutual reinforcement in natural forests, but confounding factors like canopy height and stand density may mask this relationship.