Resource pulses are a common event in agro-ecosystems. A pot experiment was conducted to assess the effects of nitrogen (N) pulses and competition on the growth of an invasive weed, Amaranthus retroflexus, and a native crop, Glycine max. A. retroflexus and G. max were planted in pure culture with two individuals of one species in each pot and in mixed culture with one A. retroflexus and one G. max individual and subjected to three N pulse treatments. The N treatments included a no-peak treatment (NP) with N applied stably across the growing period, a single-peak treatment (SP) with only one N addition on the planting date, and a double-peak treatment (DP) with two N additions, one on the planting date and the other on the flowering date. N pulse significantly impacted biomass and height of the two species across the whole growing season. However, only the relative growth rate (RGR) of A. retroflexus was significantly affected by N pulse. A. retroflexus had the greatest biomass and height in the SP treatment at the first harvest, and in the DP treatment at the last three harvests. Pure culture G. max produced the greatest biomass in the DP treatment. In mixed culture, G. max produced the greatest biomass in the NP treatment. Biomass production of both species was significantly influenced by species combination, with higher biomass in mixed culture than in pure culture at most growth stages. Relative yield total (RYT) values were all greater than 1.0 at the last three harvests across the three N treatments, suggesting partial resource complementarity occurred when A. retroflexus is grown with G. max. These results indicate that A. retroflexus has a strong adaptive capacity to reduce interspecific competition, likely leading to its invasion of G. max cropland in China.