Abstract: To study the effect of oil phase and water phase on the wall thickness in the emulsion encapsulation method, an expression of the wall thickness varying with the polymer mass fraction in oil phase and the volume ratio of polymer solution and inner water has been deduced in the ideal state. It is shown that the wall thickness increases with increasing the polymer mass fraction or the volume ratio of polymer solution and inner water. Therefore, the wall thickness can be controlled by adjusting either the polymer mass fraction or the volume ratio of polymer solution and inner water. According to the rule, the optimal parameter values are obtained by varying the parameters of oil phase and water phase in the agitation method for preparing hollow polystyrene (PS) microspheres. The experimental results show that, in order to prepare PS microspheres with 10 m to 25 m wall thickness, the PS mass fraction should be controlled in the range of 5.3% to 7.0% while the volume ratio of PS solution and inner water should be within the range of 1.6 to 2.2. Moreover, the optimal range of PVA mass fraction in outer water is between 1% and 3%.