Abstract: The trace soluble impurities in cryogenic fluids in one of the major threat that blocks the filters in pipes in the filling process.An experimental apparatus has been established for visualizing the status of crystals of water and carbon dioxide impurities in liquid nitrogen.Three fractions of water,namely 0.001%,0.01% and 0.1% in volume,were tested for observing the formation and variation of the crystals.Transient microscopic images were obtained.During the cooling pro-cess,the ice shows ignorable change as the temperature decreases.The carbon dioxide impurity forms feather shapes and gathers at temperatures between 77 to 65 K.During the rewarming process,the re-desublimation of carbon dioxide was observed,which however might be due to the ununiformity of temperature profile in the chamber.This paper further summa-rizes the ralationship between area,quantity and average size of the impurities and temperature during the cooling and the rewarming process,and reveals the existence behavior of trace soluble impurities in liquid nitrogen,which provides guidance for the design of filling pipes in aerospace applications.