Ultracold Cs2 molecules have been formed by photoassociation. Using the multiphoton ionization technique, we measure the time-evolution of expanding process in ultracold Cs2 molecule system and obtain the decay curve of photoionization signals. Based on a simple case, where the initial distribution of atomic or molecular sample is a Gaussian function of position and of velocity, we get the sample's temperature by theoretical simulation. The result shows a reasonable agreement with the result of release-recapture method in cold atom sample. This method avoids the disadvantage of detecting the weakly fluorescence and can be widely used for measuring the temperature in cold atom or cold molecule system.