N2 spectral in glass capillary by dielectrical barrier discharge at discharge frequency of 20 kHz are measured via concentration modulation spectroscopy. The spectral lines at 357.7 nm of N2 C3∏u-B3∏g and at 391.4 nm N+2 B2∑+u-X2∑+g are recorded to study the effect of discharge voltage and gas pressure. The measurement results show that the intensity increases rapidly with discharge voltage at a lower voltage range, but the speed slows at the higher voltage range, for constant gas pressure of 130 Pa; and the intensity increases backwardly with gas pressure at constant discharge voltage of 6.4 kV. Furthermore, a theoretical model is proposed according to electronic excitation function and ionization function, which can basically interpret the relation between spectral intensity and discharge parameters, fit the experimental data well, and the correlation coefficient is over 0.9. The mechanism of the dependence of the emission spectral intensity on discharge voltage and gas pressure in the plasma discharge was revealed further.