Ar plasma was produced by using 1 064 nm laser at atmospheric pressure, the emission spectrum was measured, and 35 Ar Ⅰ and Ar Ⅱ emission lines from the 15 upper levels were selected to make calibration for spectrometer response based on the branch ration method. In addition, the intensity response of the spectrometer was calibrated by using deuterium lamp and tungsten halogen standard light source. The calibration coefficients of intensity response for spectrometers with different gratings were obtained. The calibration coefficient of the intensity response obtained by the branch ratio method was in good agreement with that by the standard light source, and the maximum relative error is 54%. The calibration method in this paper can be used as a reference for the on-line calibration of spectral measurements in some large-scale fusion devices.