A solar flux measurement method for the concentrated solar irradiance based on indirect measurement principle is proposed. This method does not require additional thermal flux meters or energy sensors to calibrate the camera, which simplifies the measurement process. It takes images of the sun and the concentrated solar irradiance beam by using the same camera and the same camera settings. Combining with the direct normal irradiance of the sun, it can figure out the scale factor between the pixel grey values of the camera and the flux density. And then, it obtains the flux distribution of the beam. The measurement principle of this technique is introduced in detail, and a testing platform is built. The flux map of a piece of small heliostat is measured, and the feasibility and correctness of this method are verified. The experimental results indicate that the relative error of the peak flux measured by this method is less than 5%, and the repeatability error is about 1%.