Abstract: The rock is quasi-brittle material. Under the condition of uniaxial compression, stress increases with the increase of axial strain firstly, when the stress reaches the maximum value, the stress begins to decline, specimen presents strain softening behavior. To research the effect factors of stress-strain softening segment in single axis compression of rock specimen, the slope analytic expression of stress-strain curve softening segment during single axis compression of rock specimen are got based on displacement method, and FLAC^2Dsoftware are used to analyze uniaxial loading simulation results including angle α formed by different shear zone and principle stress direction and rock specimen with different aspect ratio, and the analytic expression is verified by the result.