A new method measuring the quality of dairy products is proposed. The low-coherence dynamic light scattering setup consists of the combination of the low-coherence source and Michelson interferometer and the traditional dynamic light scattering technology. By detecting the variation of the field autocorrelation function of the single backscattered light, the deterioration of the milk can be discriminated on-line. The autocorrelation function of the single backscattered light field intensity from milk saved in refrigerator and room temperture was dectected, and the CONTIN algorithm was used to analyze the experimental data. The experimetnal results show that the autocorrelation function of the single backscattered light field intensity from milk saved in room temperture did not change in the anterior five days, but the autocorrelation function varied violently in posterior two days, indicating that the milk is deterioration and the particle size distribution is wider. It proves that the method can measure the quality of dairy products.