The disclosure provides a method for determining a slope slip plane with a gently-inclined soft interlayer, including: S1, determining a curve formed with a slip arc of a trailing edge tearing plane, a soft interlayer plane and a slip arc of a leading edge shear opening as a slope slip plane; S2, calculating a slip plane stability coefficient; S3, determination of a position of the gently-inclined soft interlayer plane: if the slip plane stability coefficient is less than 1 but close to 1, determining that the position of the slope slip plane is accurate; otherwise, moving the position of the soft interlayer plane and repeating steps S1 and S2, until the slip plane stability coefficient is less than 1 and close to 1. The method is simple, and has a high accuracy for determining a non-circular slip plane with a soft interlayer as a bottom slip plane.