The invention relates to a cellulose fiber fabric cationic modification and salt-free alkali-free dyeing method. The method comprises following steps: adding soft water into a gas-liquid dyeing machine, adding cellulose fiber fabrics and a cationic modifier into the dyeing machine, starting the dyeing machine; adding sodium hydroxide into the dyeing machine from a material dissolving cylinder, adjusting the water level, running the dyeing machine, raising the temperature, discharging the modification liquid, washing the fabrics by a water solution of glacial acetic acid at a room temperature,taking out the fabrics, dehydrating the fabrics to obtain cation modified fabrics; adding soft water, adding cation modified cellulose fiber fabrics, dissolving active dyes, injecting the dyes into the dyeing machine, raising the temperature to the dyeing temperature, maintaining the temperature, discharging dyeing liquid, taking out the fabrics, and dehydrating and drying the fabrics. In the gas-liquid dyeing machine, cellulose fiber fabrics are modified under conditions: medium-low temperatures and a small bath ratio; the using amount of modifier is little; the surface and interior of fabrics are uniformly modified; the modified fabrics can be dyed without salt or alkali, furthermore, the dyeing time is short, the dyeing process is simple and controllable, the color is uniform and bright, and the color fastness meets the standards.