To provide new research direction for the subsequent casing damage mechanics research, the paper presents a new method which uses porosity as damage variable to describe mudstone creep. The authors obtained the relationship between macroscopic damage variables and the mesoscopic damage variable, established mudstone creep damage constitutive model based on mesoscopic damage variable. In this paper, the authors take Changqing oil field stone box group mudstone rock specimen as an example, getting stress-strain constitutive relationship of mudstone creep damage. After comparing theoretical calculation with practical results, the maximum error was 9.204%, and the calculated results are in agreement with the actual ones.