According to the relation between intensity Autocorrelation Function (ACF) and normalized light field ACF,the effect of different baseline values on the results measured by using Photon Correlation Spectroscopy (PCS) was presented.The cumulant method which was used to inverse the size and polydispersity of nano-particle in PCS was analyzed.Three types of materials with five different sizes were measured by using PCS and the results showed that when the baseline methods were different,the value of baselines and the results of the effective diameter and polydispersity inversed by cumulant method were different,too.The reason was analyzed theoretically.It is pointed out that the autoslope method of acquiring baseline is the appropriate measured method.