Relationship among equilibrium moisture content (EMC), temperature and pressure was established under vacuum condition, and EMC regression equation was constructed based on experimental data, with Chinese fir (Cunninghamia lanceolata (Lamb.) Hook) as Experimental material, and MC was measured by oven-drying method. The experimental temperature range was from 40 to 90°C at the interval of 10°C, and pressure range was from 0.02 to 0.08 MPa at the interval of 0.02 MPa. EMC Comparison of Chinese fir and paper wafer sensor showed: EMC of paper wafer sensor changed in the same trend of solid wood. A regression equation was presented and the calculated EMC value was highly correlated with experimental data. This regression equation can be used to predict EMC under vacuum, especially for automatic control system of wood drying under vacuum.