Moderator cell of the cold neutron source (CNS) is an important component of China Advanced Research Reactor (CARR). It is a critical problem to keep the void fraction of two-phase hydrogen in the moderator cell to a specified range. Many mockup tests should be carried out for the purpose of the proper design of CARR. However, it is impossible to use hydrogen as test fluid because of the safety and economy consideration, therefore, experiments in order to select a suitable working fluid to study the void fraction were carried out. An annular vessel having the same size with the actual moderator cell was used as test section. Deionized water, alcoholic solution, sucrose solution and sodium chloride solution with different concentrations were used as working fluid to find out the influences of physical properties, such as density, viscosity and surface tension, of two-phase mixture on void fraction. The tests proved that the ratio of surface tension to density of liquid phase has great influence on void fraction, the larger the ratio, the smaller the void fraction. So Freon 113 can be used as working fluid to simulate the void fraction in the two-phase hydrogen thermosiphon loop in the CNS of CARR.