In order to investigate whether the radiometric responsivity of the detector in large-scale dynamic range is constant under the illumination of monochromatic light, a laser-based linear measuring device was established, and the measurement uncertainty of the device was evaluated. According to the dual optical path superposition method, the linearity of the photodetector was measured in both the power mode and the irradiance mode. For the power mode, the nonlinear coefficient was measured with the laser power varying over 6 orders of magnitude,while for the irradiance mode, the laser was guided into an integrating sphere to output the optical radiation, and the nonlinear coefficient of the detector under 3 orders of magnitude changes was measured. Results show that when the current of the photodetector is varied from 0.2 nA to 0.2 mA in power mode, the nonlinear fact of the detector is less than ±0.02%. When the current of the detector is changing from 200 nA to 0.2 nA in irradiance mode, the nonlinear factor is also less than ±0.02%. It can be seen that the laser linear device can realize two modes of power and illuminance measurement, and meets the measurement of at least 6 orders of magnitude or more.