1, Sichuan basin covers an area of about 260,000 square kilometers. The outline of Sichuan Basin is generally approximate to a rectangle, shaped like an envelope, and tilted to the right on the Asian continent, so it is called an envelope basin by geographers. Yunyang (near Wanzhou) in the east, Xuyong in the south, Ya 'an in the west and Guangyuan in the north of the envelope basin. Suining, the geometric center city of the envelope basin, Nanchong, the central city of the north-central part, and Anyue, the inner corner of the south-central part.

2. The Sichuan Basin can be clearly divided into two parts: the marginal mountain and the bottom of the basin. The marginal mountain is mostly middle mountain and low mountain. The elements of the landscape are obviously transitional, for example, the composition of animals and plants has penetrated the components of Central China, Southwest China, Qinghai-Tibet Plateau and North China respectively. Generally, there are 2 ~ 5 vertical natural zones in marginal mountain areas from bottom to top. The marginal mountainous area is the base of various economic trees and timber forests in Sichuan. Soil erosion is serious in agriculture. There are many hills, low mountains and plains at the bottom of the basin. The surface composition is new and single, sandy mudstone and Quaternary sediments. The climate belongs to the middle subtropical zone, and the heat is much higher than that in the marginal mountains, but the precipitation is not as good as that in the marginal mountains. Vegetation is replaced by artificial secondary forest trees, and the coverage rate is low, the cultivated land is contiguous, and drought and flood in summer are the biggest disasters in this area.