In order to explore the key factors of maintaining the understory vegetation diversity in forest light condition and canopy structure in the alpine region of Qinghai, we observed five typical plantations in Datong County, Qinghai Province, including Picea crassifolia forest (Ⅰ), Populus cathayana forest (Ⅱ), Larix principisrupprechtiPicea crassifolia mixed forest (Ⅲ), Betula platyphyllaPopulus cathayana mixed forest (Ⅳ), and Betula platyphyllaPicea crassifolia mixed forest (Ⅴ), to study the characteristics of the canopy structure and light conditions and their effects on understory vegetation. Data were collected using a canopy analyzer and combined with field investigations. The results showed that: （1） the canopy openness of stands Ⅰ and Ⅲ was significantly lower than that of stands Ⅱ, Ⅳ and Ⅴ (P<0.05). The order of leaf area index of each forest was Ⅲ, Ⅰ, Ⅱ, Ⅴ and Ⅳ. The canopy openness of broadleaved forest was larger than that of coniferous forest, but its leaf area index was lower than that of coniferous forest. The understory direct PPFD, understory diffuse PPFD and total understory PPFD of stands Ⅱ and Ⅳ was significantly higher than that of those of forest stands Ⅰ, Ⅲ and Ⅴ (P<0.05), in which total understory PPFD and understory diffuse PPFD were Ⅱ>Ⅳ>Ⅴ>Ⅰ>Ⅲ, the understory direct PPFD was Ⅱ>Ⅳ>Ⅰ>Ⅲ>Ⅴ; K of the stands Ⅱ, Ⅳ and Ⅴ was significantly higher than Ⅰ and Ⅲ; broadly expressed as broadleaved forest > coniferous forest.（2）The canopy openness of each stand was very significantly positively correlated with the understory light indices (P<0.01), the leaf area index was significantly negatively correlated with the understory light indices (P<0.01); and it had the strongest control ability for understory diffuse PPFD. Canonical correlation analysis showed that canopy openness of pure forest had greater contribution and interpretation of canopy structure than leaf area index and mean leaf angle. The leaf area index of mixed forest had greater influence on the understory light than that of pure forest. （3） The understory species ShannonWiener index (H) and Patrick index (P) of mixed forest were higher than that of pure forest; The ShannonWiener index (H) and Patrick index (P) of the undergrowth herb layer were positively correlated with canopy openness and understory light radiation and negatively correlated with leaf area index (P<0.05); there was a very significantly negative correlation between Pielou index (J_sw) and mean leaf angle (P<0.01) and significantly positive correlation between Pielou index (J_sw) and understory diffuse PPFD (P<0.05). It is more conductive for the maintenance of understory vegetation diversity to replant associated tree species in the arbor layer, gradually adjust to multitree mixed forest and increase canopy structure and the understory heterogeneity.