A stable diffuse large-area air plasma with low gas temperature is developed under different dielectric thicknesses by using a needle array-plate electrode configuration in air at atmospheric pressure, which is excited by a bipolar nanosecond pulse power with about 20 ns rising time. The images of the diffuse discharge, electric characteristics, and the optical emission spectra emitted from the diffuse discharge are obtained under different dielectric thicknesses. The discharge area, the average power, the power density, and the emission intensities of N2 (C3Πu → B3Πg, 0-0) and N2+ (B2Σu+ → X2Σg+, 0-0) of the nanosecond dielectric barrier discharges are investigated under different dielectric thicknesses based on the waveforms of discharge voltage and discharge current and the optical emission spectra. The gas temperature of plasma is determined to be approximately 385±5 K by measuring the optical emission spectra of the N2+ (B2Σu+ → X2Σg+, 0-0), and decreases gradually with increasing dielectric thickness. Consistently, an area of approximately 2000 mm2 plasma region under 7 needle electrodes can be formed at 1 mm thick dielectric layer.