Description: We monitored the water quality and hydrological conditions of a horizontal subsurface constructed wetland (HSSF-CW) in Beijing, China, for two years. We simulated the area-based constant and the temperature coefficient with the first-order kinetic model. We examined the relationships between the nitrogen (N) removal rate, N load, seasonal variations in the N removal rate, and environmental factors—such as the area-based constant, temperature, and dissolved oxygen (DO). The effluent ammonia (NH4+-N) and nitrate (NO3−-N) concentrations were significantly lower than the influent concentrations (p 〈 0.01, n = 38). The NO3−-N load was significantly correlated with the removal rate (R2 = 0.96, p 〈 0.01), but the NH4+-N load was not correlated with the removal rate (R2 = 0.02, p > 0.01). The area-based constants of NO3−-N and NH4+-N at 20 °C were 27 ± 26 (mean ± SD) and 14 ± 10 m∙year−1, respectively. The temperature coefficients for NO3−-N and NH4+-N were estimated at 1.004 and 0.960, respectively. The area-based constants for NO3−-N and NH4+-N were not correlated with temperature (p > 0.01). The NO3−-N area-based constant was correlated with the corresponding load (R2 = 0.96, p 〈 0.01). The NH4+-N area rate was correlated with DO (R2 = 0.69, p 〈 0.01), suggesting that the factors that influenced the N removal rate in this wetland met Liebig’s law of the minimum.