This paper presents a multi-scale damage model for quasi-brittle materials, in which the macroscopic damage is considered as the consequence of micro-cell cracking process. The stochastic phase-field theory is employed to describe the crack initiation and propagation in the micro-scale, while the continuum damage model is applied for the damaged continua in the macro-scale. To characterize the micro-cell, a proper method of determining the microscopic random-field parameters is developed. As determined using micro-cell analysis, the homogenized stress-strain relation and damage evolution law exhibit a strong size effect. Thus, a set of scaling formulas is developed to quantify the damage evolution relationship of micro-cells with different sizes, which can properly characterize the interations among various scales. Finally, two numerical examples, including a three-point bending notched beam and a four-point bending reinforced concrete beam, are presented to verify the proposed method.