The singular value decomposition method is used to study the characteristics of light energy coefficient matrix of a laser particle size analyzer, and the sensitivity parameters are defined to express the variance of particle size distribution with the light energy distribution. The measurement upper limit under certain parameters is obtained. An analytic formula which discloses the relationship between the measurement upper limit and the physical parameters is derived, which shows that the measurement upper limit of the instrument is determined by the minimum acceptance angle of this instrument for the scattered light under the condition of constant incident wavelength. The experimental results verify the correctness of this formula.