Abstract: Stress analysis is done to the soil arch between piles.It is based on existing research of anti-slide pile spacing.Pile spacing is calculated by two conditions.They are side resistance and soil arch strength.Unified strength theory is introduced to analyze soil arch strength.On the basis of this analysis,the front and posterior arch roof and arch foot are respectively judged that the soil is or is not in critical state.Then the values of three pile spacing can be obtained.The corresponding minimum pile spacing is taken as the design one.The method is suitable for slide thrust distribution with the form of either rectangle,or triangle or trapezoid.Soil self-weight stress can be also considered.It can be simultaneously applied to the anchored anti-slide pile spacing.Two calculation examples are analyzed by the presented method.The comparison between the calculation results and the existing or design results shows that the effect of proposed method is satisfactory.When specific calculation is done,the suitable value of unified strength parameter b is between 0.2and 0.7.When slide thrust is rectangular distribution,b is about 0.3.For triangular distribution,b is about 0.6.For trapezium distribution,b lies between 0.3and 0.6.