Abstract: The liquid volume entrained on air-liquid paraffin and air-liquid paraffin-catalyst C301 system was calculated on the basis of the vapor pressure of liquid paraffin which is an inert medium for dimethyl ether (DME) synthesis in slurry bubble column reactor (SBCR). The effect of operation condition(temperature, pressure and superficial velocity) and dimension of equipment(obliquity of separator) on liquid entrainment was studied. The entrainment in three-phase bed reactor is in contrast with that in two-phase bed reactor, which indicates entrainment in three-phase bed can be regarded as that in two phases bed approximatively. Furthermore, the mechanism of liquid entrainment and the efficiency of separation were studied. The mechanism of carrying out liquid was also discussed. The main factors of entrainment are break of bubble and increase of energy in the liquid.