In order to establish a calculation model for the 28d compressive strength of cement asphalt mortar(CA mortar), the significance of the influence of various mix proportions to the strength were verified by orthogonal experiment. By SEM, the micro structures were observed to study the mechanism of the influence of mix proportions to the strength. Refering to the micro mechanics theory for concrete, a theoretical model for the 28d strength was founded based on porosity and hydration degree. Results show that the theoretically calculated results and the real strength are in good consistency in larger range.