In order to measure both large and small particles, based on the analysis of the pulse signal of the spherical particle irradiated by a thin beam, a method for extracting the equivalent particle size information was proposed. This method combines the amplitude and duration of the pulse signal by the integral of the pulse signal, which makes the equivalent particle size information extracted from the pulse signal more accurate. The ideal pulse signal was simulated and experiment was made by using standard polystyrene latex particles. The results show that the method can effectively extract the particle size information of large and small particles, which broaden the detection range. In addition, the effect of flow velocity and noise on the extraction of the particle size was analyzed. When the flow rate is less than 10%, the measurement error is less than 5.4%, and when the noise is less than 1%, the measurement error is less than 0.5%.