Abstract: The damage caused by weeds in fields affects the growth of Codonopsis pilosula seedlings, and the use of herbicides damages C. pilosula seedlings and has a poor effect. To explore available ways to control weeds in C. pilosula seedling fields in the main production areas of Gansu Province, dazomet was used to fumigate the soil before sowing (F) while using non-fumigated field as the control (CK). The dynamics and differences of weed communities in the seedling fields were systematically studied. The results showed that 1) during seedling cultivation, the similarity in weed community between F and CK fields was 83.3%, and the average WCs between weed communities in emergence stage and seedling growth stage in the fumigated field was 18.9% lower than that in the CK field. Fourteen species of weeds (10 families and 14 genera) occurred in the CK field, of which five (including two annual weeds of Myosoton aquaticum and Setaria viridis, and three perennial weeds of Cirsium arvense, Convolvulus arvensis, and Sonchus brachyotus) were dominant. Five weed species (Polygonum aviculare, S. viridis, S. brachyotus, Malva cathayensis, and Hypecoum leptocarpum) were reduced in the seedling stage of C. pilosula, and three weed species (P. aviculare, Chenopodium glaucum, and H. leptocarpum) were reduced during the whole growth of C. pilosula in the F field, which on an average decreased by 39.8%. 2) Soil fumigation decreased the diversity of primary weed communities in the early stage of C. pilosula seedlings, in which the weed richness index decreased by 0.68, Shannon-Wiener index decreased by 0.50, and Simpson’s index decreased by 0.36. Soil fumigation also reduced the population abundance of malignant weeds and significantly inhibited its regeneration. 3) Soil fumigation significantly reduced the occurrence density of primary weeds, reducing 477 primary weed plants per square meter in early June. In the F field, the density of primary weeds of seven families was less than 5 plants∙m⁻², and the distribution was uniform. Among 10 primary weeds in the CK field, Caryophyllaceae had the highest density, and it accounted for 67.6% of the total number of primary weeds in the field, which was significantly higher than that of other weeds. Polygonaceae, Amaranthaceae, and Gramineae accounted for 22.5%, 5.3%, and 2.2%, respectively, while the other six families in total accounted for only 2.4%. The primary amount of Caryophyllaceae, Polygonaceae, and Gramineae weeds was larger in the non-fumigated field. Soil fumigation reduced 143 regenerated weeds per square meter from late June to Oct and, thus, could reduce the density of regenerated weeds. The average weed density and fresh biomass significantly decreased by 89.0% and 77.5%, respectively. It obviously controlled the early sexually reproducing weeds, such as M. aquaticum and Fagopyrum gilesii, and the late perennial facultatively reproducing weeds such as S. brachyotus and Convolvulus arvensis. These results indicate that soil fumigation with 98% dazomet before sowing can effectively control and alleviate the damage caused by weeds in C. pilosula seedling fields. However, the perennial deep-rooted weeds should be controlled early.