The paper studies the measuring time for Au particle size measurement at different concentration using the method of Photon cross-correlation spectroscopy (PCCS).The basic principle of measuring the size of nano-particles by PCCS is introduced.We measured gold particles with an average diameter of 24.5 nm at different concentrations with different measuring times by PCCS.The measuring results show that for the same measuring time,the higher the concentration is,the smaller the standard deviation of the measurement is;for the same suspension concentration,the longer the measuring time is,the smaller the standard deviation of the measurement is.Relationship between the standard deviation and the measuring time at difference concentration is obtained.According to the measuring accuracy needed,the measuring time for certain concentration can be is decided by the fitting curve.