By analyzing the effects of different straw mulching amounts and tillage methods on the soil porosity, soil water retention property, soil water supply capacity and soil water effectiveness in 0~20cm, 20~40cm and 40~60cm soil layers, the mechanism of straw mulching and tillage methods on soil water characteristics in black soil region was revealed. The result showed that soil water retention property, soil water supply capacity and soil water effectiveness were closely related to soil porosity. Under conventional tillage treatment, the soil total porosity, soil capillary porosity, soil water content under the same soil water suction, specific water capacity and available water content under straw mulching treatment were higher than those under bare soil in 0~20cm, 20~40cm and 40~60cm soil layers. Under straw mulching treatment, the soil total porosity, soil capillary porosity, soil water content under the same soil water suction, specific water capacity and available water content under no-tillage treatment were higher than those under conventional tillage treatment in 0~20cm, 20~40cm and 40~60cm soil layers. No-tillage with straw mulching treatment affected soil porosity, soil water retention property, soil water supply capacity and soil water effectiveness. Under no-tillage treatment, soil total porosity, soil capillary porosity, soil water content under the same soil water suction, specific water capacity and available water content were increased with the increase of straw mulching. Soil porosity, soil water retention property and soil water supply capacity were decreased gradually with soil depth. The most suitable straw mulching and tillage method in the study area was no-tillage with 150% straw mulching.