Excessive use of nitrogen (N) fertilizers in agricultural systems increases the cost of production and risk of environmental pollution.  Therefore, determination of optimum N requirements for plant growth is necessary.  Previous studies mostly established critical N dilution curves based on aboveground dry matter (DM) or leaf dry matter (LDM) and stem dry matter (SDM), to diagnose the N nutrition status of the whole plant.  As these methods are time consuming, we investigated the more rapidly determined leaf area index (LAI) method to establish the critical nitrogen (N_c) dilution curve, and the curve was used to diagnose plant N status for winter wheat in Guanzhong Plain in Northwest China.  Field experiments were conducted using four N fertilization levels (0, 105, 210 and 315 kg ha⁻¹) applied to six wheat cultivars in the 2013–2014 and 2014–2015 growing seasons.  LAI, DM, plant N concentration (PNC) and grain yield were determined.  Data points from four cultivars were used for establishing the Nc curve and data points from the remaining two cultivars were used for validating the curve.  The N_c dilution curve was validated for N-limiting and non-N-limiting growth conditions and there was good agreement between estimated and observed values.  The N nutrition index (NNI) ranged from 0.41 to 1.25 and the accumulated plant N deficit (N_and) ranged from 60.38 to –17.92 kg ha⁻¹ during the growing season.  The relative grain yield was significantly affected by NNI and was adequately described with a parabolic function.  The N_c curve based on LAI can be adopted as an alternative and more rapid approach to diagnose plant N status to support N fertilization decisions during the vegetative growth of winter wheat in Guanzhong Plain in Northwest China.