Clarifying the effects of different paddy-upland rotation patterns on the characteristics of weed communities in upland crop season in West Sichuan Plain can provide an important theoretical guidance for weed efficient control. Based on a long-term experiment of five paddy-upland rotations, including garlic-rice (GR), wheat-rice (WR), broad bean-rice (BR), rape-rice (RR), and potato-rice (PR), the weed community structure and species diversity characteristics, including the weed species, density, plant height, biomass, and other related indicators of different paddy-upland rotations were investigated by ecological method in the upland season of 2022. The results showed that: (1) under the five rotation systems, the density of weeds was ordered by WR>BR>RR>GR>PR, while the biomass order was WR>BR>GR>RR>PR. Species diversity was ordered by GR>BR>RR>PR>WR, species evenness was ordered by GR>PR>BR>RR>WR, and the community dominance was ordered by WR>PR>RR>BR>GR. These results indicated that the stability of weed community was the highest under GR and lowest under WR, but the degree of damage of dominant species was the highest under WR, and also, the quantity of weed species under RR was small and not easy to harm. (2) The Bray-Curtis similarity index of weed communities in the five paddy-upland rotations was high, which was between 0.69 and 0.84. Annual monocotyledonous weeds were the dominant weed life form under WR, while that of the other four systems were annual dicotyledonous weeds. (3) The kind of dominant weed under WR was the least, which was Polypogon fugax Nees ex Steud, and its importance value was 59.53%. BR had the most kinds of dominant weed species, which contained Polypogon fugax Nees ex Steud, Rorippa indica (L.) Hiern, Ranunculus japonicus Thunb, and Mazus japonicus (Thunb.) O. Kuntze. The dominant species under GR was Sonchus wightianus DC and Galium spurium L. There were three dominant weeds under RR and PR, both of which included Polypogon fugax Nees ex Steud and Rorippa indica (L.) Hiern. To ensure effective weed management in upland crop systems, it is important to tailor the control measures to the specific paddy-upland rotation system in question. For instance,in GR and PR systems, the emphasis should be on managing fast-growing weeds such as Galium spurium L. In WR system, monocotyledonous weeds like Polypogon fugax Nees ex Steud should be the main target for control measures. Similarly, in BR and RR systems, the effective weed management should focus on controlling both monocotyledonous and dicotyledonous weeds with strong fecundity.