In order to illustrate the effect of biochar on soil retention ability in dry farmland, this study measured the soil bulk density, soil porosity, soil moisture constants and the soil water retention curve of the soil with different biochar addition through setting located experiment in typical dry farmland of the Loess Plateau. The results showed that biochar adding reduced soil bulk density, and increased soil porosity. With the increasing of the biochar adding, the reduction range of soil bulk density and the increasing extent of soil porosity were both enhanced. The change of soil structure was the most obvious when the adding amount of biochar reached 50 t/hm2. Soil bulk density of 0—5 cm, 5—10 cm and 10—30 cm soil layers decreased by 7.01%, 9.91% and 16.60% compared with control group, respectively, and soil capillary porosity increased by 19.47%, 21.02% and 29.94% compared with control group, respectively. The biochar adding increased soil saturated soil water content, soil field capacity and soil effective moisture content. With the increasing of the biochar adding, all the soil moisture constants showed rising trends, but the increasing extent decreased when the biochar amount reached 40 t/hm2. This study suggests that the addition of biochar improved the water retention ability of dry farmland, but the retention ability of soil tended to be stable when the adding amount of biochar reached 40 t/hm2.