Abstract: The study site was located in Saihanba mechanical forest farm of Hebei province, through the research on the effects of stand density on understory vegetation diversity of Larix principis-rupprechii plantation by using of sample plot investigation method, the result showed that the effects of stand density on Simpson index D of shrub and Shannon-Wiener index H of herb are biggest in each of the diversity index of understory vegetation of Larix principis-rupprechii plantation. There were a certain extent effects of stand density on families and genera compositions, species richness, species diversity, species evenness of understory vegetation of Larix principis-rupprechii plantation and they showed decreasing trend in different degree with increasing of stand density.