A pot experiment was conducted to compare the chlorophyll fluorescence (ChlF) and growth performance between invasive Amaranthus palmeri S. Watson and its native congener Amaranthus tricolor L. under 4 nitrogen (N) supply patterns (control, LN, HN and PN) in monoculture or co-culture. The patterns followed a gradient of increased temporal variability of N supply. The Fv/Fo, QYmax, and Fv’/Fm’ of A. palmeri were notably higher than A. tricolor in monoculture. However, in co-culture, invaders enjoyed these advantages under the CK and LN treatments. The above ChlF parameters of invaders in co-culture significantly decreased except it under the PN treatment. The invader in coculture possessed the minimal NPQ and maximal 1-qP under the HN and LN treatment respectively. A. palmeri also had greater total biomass mainly attributed to its pronounced taller plant height, which contributed super competition abilities to it over congeners. Although the N input improved A. palmeri’s growth in co-culture, its ChlF declined under relatively low N variability patterns (LN and HN treatments). Only the higher N variability (PN treatment) promoted invaders’ ChlF and growth simultaneously. Results indicated the invasion of A. palmeri would be promoted with the increased N supply variability.