The working fluid filling rate of a special separate type heat pipe is studied experimentally. Acetone is taken as the working fluid. Its filling rate is changed between 32% and 179%, while the velocity of air and heating power for the system are fixed. In these experiments, the wall temperature of the separate type heat pipe exchanger and the outlet air temperature of the upper and lower wind tubes are measured. The heat transfer characteristics in different working states are analyzed respectively and, in this condition, the optimum filling rate is found as from 70% to 114%.