For in-line measurement of particle size and concentration based on the light fluctuation method, the intensity of incident light must be pre-detected. However, it is very difficult to measure the incident light intensity during the in-line measurement because the incident light intensity is easily affected by many factors. A novel method based on Gregory′s light fluctuation method is proposed. The relation among the intensity of incident light, the ratio of particle concentrations at different time, the standard deviation and the mean value of the intensity of transmitted light is established. With those measurable parameters the intensity of incident light may be obtained. Experimental results agree well with the pre-detected value of the intensity of incident light, and the maximum deviation is less than 1.1%.