Nanocarbon has been widely used as a new material in new energy and ecological restoration fields due to its unique physical and chemical properties. In recent years, as its functions are continuously explored, nanocarbon has started to be put into the agricultural field and become a new research hotspot. However, it is still unclear how the addition of nanocarbon affects soil available nutrients and crop yield, and further research is needed. Nanocarbon was used as a fertilizer synergist in different fertilizer formulations to study its effects on soil fast-acting nutrients, agronomic traits and yield of rape under field conditions, thus providing a theoretical basis for the efficient use of nanocarbon in agriculture. The field trial was conducted in a greenhouse in Doudian Township, Fangshan District, Beijing, with a small oilseed rape crop, variety "Guoxia 1". Four treatments were used: chemical fertiliser (CF), chemical fertiliser + nanocarbon (CF + NC), organic fertiliser (OM) and organic fertiliser + nanocarbon (OM + NC). Soil fast-acting nutrients, rape plant height, leaf area and yield were measured at different fertility stages of small oilseed rape. The results showed that: 1) Compared with CF and OM, CF+NC and OM+NC treatments increased the soil fast-acting nutrient content and increased the soil fast-acting potassium content by 37.3% and 27.4%, soil ammonium nitrogen content by 35.5% and 35.5%, and soil nitrate nitrogen content by 52.0% and 49.9%, respectively, at the peak of small oilseed rape growth. 2) The addition of nanocarbon promoted the growth of small oilseed rape. Compared with CF and OM, CF+NC and OM+NC treatments increased the height of small oilseed rape plants by 9.7% and 5.6%, leaf area by 13.9% and 12.9%, dry weight by 15.6% and 14.7%, and yield by 19.8% and 18.0% at harvest, respectively. Compared to fertiliser applications alone, the application of NC with inorganic and organic fertilisers significantly increased the quick-acting nutrient content of the soil, promoting the growth and development of small oilseed rape and improving the yield.