We developed a model of water evaporation rate from wood surface under vacuum condition using birch as material with cross-sectional area of 20 mm×20 mm and length of 100, 150, 200, 250 and 300 mm.The internal water moving rates of specimen were studied under the temperature of 60, 75 and 90℃ and absolute pressure of 0.02, 0.04, 0.06, 0.07 and 0.08 MPa. The results show that the surface water evaporation rates were much faster than the internal water moving rates, and the ratio of the former to the later varied from 10 to 150. In addition, the relations of the drying rate with temperature and absolute pressure under different conditions were deduced based on the experimental results. Compared with the model of surface water evaporation rate, we can obtain the ratio of the wood surface water evaporation rate to the internal water moving rates from the regression equation. Finally, the optimum formula between temperature and absolute pressure was deduced according to the maximal ratio of the surface water evaporation rate to the internal water moving rate, which could cause drying defects during vacuum drying.