In the visible light region,the ill- condition problem in particle size measurement based on spectral extinction technique is analyzed.By analyzing the results of objective function and inversion results,the influence of the ill-condition problem on spectral extinction method is discussed.The experiments are performed with standard polystyrene particles and the particle size distribution is inversed.Both simulation and experimental results show that the ill-condition problem in spectral extinction measurement is serious for the submicron particles.The situation of objective function extremum and inversion results are sensitive to error,leading to the poor stability and accuracy of the inversion results.To ensure the accuracy of the particle size measurement,measurement error should be minimized.In contrast,the situation of objective function and inversion results for micron particles is much better than for submicron particles.