Populus simonii Carr. is a species widely planted on the Loess Plateau. The results of the study show that the growth of P. simonii Carr. is constrained by spatial site conditions. Its growth reached the highest in gullies where soil water conditions was better than in other sites. On the same slope, the P. simonii Carr. grows better in lower part than in middle and upper parts. The height growth reached 12.67m in lower part while reached 5.70m and 5.21m respectively in middle and upper parts. The diameter at the base and chest height, the height of the lowest branch and the canopy growth showed the same trend. With the gully bottom as the reference site, the growth of P. simonii Carr. was evaluated. The height growth reached 77.53%, 34.88% and 31.88% of the potential growth in lower, middle and upper parts respectively while the growth of the diameter at chest height reached 75.59%, 39.51% and 33.36% of potential growth respectively. When mixed with Hippophae rhamnoides L.subsp., the P. simonii Carr. showed better growth than planted in single species. The analysis of biodiversity showed that the mixed forest of P. simonii Carr. and Hippophae rhamnoides L.subsp.had the higher biodiversity values and more evenly distributed species, and had a higher ecological benefit. These results showed that P. simonii Carr. can be planted only in gullies where soil water conditions were better, and can not be planted in large scale, especially in middle and upper parts of the slopes.