A simple and practical technique is studied to develop a near- infrared nonlinearity coefficient measurement for optical detector. Two infrared LEDs are used, and two source meters are adopted to control the LEDs and their driving currents. Using flux-addition method, the proposed method achieves the measurement of nonlinearity coefficient within four orders of magnitude at certain wavelength, which is determined by the emission peak of LED. The 940 nm LED is used as an example. The room temperature is controlled and the infrared LEDs as the light sources are quite stable. When the ignition time ranges from 1 min to 10 min, the relative standard deviation of the radiation flux is not greater than 0.022%. The reproducibility of the LED flux is also studied. The LED is turned on and off 10 times, and the relative standard deviation of the radiation flux does not exceed 0.044% at different flux levels. The measurement uncertainty is discussed and evaluated. The expanded measurement uncertainty of the calibration results is 0.07% (k=2).