Background: Littleseed canarygrass (Phalaris minor Retz.) is one of the most troublesome invasive weeds infesting winter crops in Yunnan Province, China. Our previous study found that rapeseed (Bassica napus L.) was a logical candidate crop to control littleseed canarygrass in agroecosystems. Nitrogen (N) could impact plant community composition by altering competitive interactions, however, the effects of different N regimes on weed control efficacy of rapeseed were unknown. Here, we report the effects of different N regimes on the control efficacy of littleseed canarygrass by rapeseed and their competitive effects and photosynthetic characteristics. Results: The results showed that the rapeseed yield and its control efficacy on littleseed canarygrass were significantly affected (P<0.05) under different N regimes, and the control efficacy of littleseed canarygrass by rapeseed increased first and then decreased with the increase of basal nitrogen rates, while increasing topdressing N rates increased control efficacy of littleseed canarygrass by rapeseed only. In fact, yield and weed control efficacy of rapeseed was most ideal when both basal and top-dressing N was 90 kg·ha-1. We also found that N significantly impacted the competitive ability of rapeseed to littleseed canarygrass and rapeseed had a highest competitive ability when both basal and top-dressing N was 90 kg·ha-1. With the increase of basal nitrogen rates, competitive balance index (CB) of rapeseed increased first and then decreased, but which gradually increased with increasing of topdressing N rates. Our research also showed level and period of N application had a significant effect (P<0.01) on the photosynthetic rate (Pn) and chlorophyll content (Chl) of both rapeseed and littleseed canarygrass. Under the same N application regime, the Pn and Chl of littleseed canarygrass were higher than that of rapeseed in December, while the Pn and Chl of rapeseed was higher than that of littleseed canarygrass in February. Our study indicated that photosynthetic characteristics of rapeseed and littleseed canarygrass in different growth stages differ in their sensitivity to N regimes, creating a dynamic competitive relationship. Conclusions: Together, our results demonstrated that rational application of fertilizer N could help rapeseed produce higher yields and greater weed control efficacy, suggesting that future modeling or experimental studies on utilizing crops to control invasive weeds should carefully consider both timing and placement of N.