To solve the problem of the reduction of compressive strength of aluminate cement（HAC） used in high CO2 content oil well cementing, the influences of silica fume（SF）, slag（BFS）, fly ash（FA）, sodium hexametaphosphate(SHP) and their combined use on the compressive strength of aluminate cement were investigated. According to the experimental results, the combined admixture（ASS） composed of 100%（by mass） sodium hexametaphosphate and 75%（by mass） silica fume was optimized for modification of aluminate cement, and on this basis the impacts of carbon fiber（CF）, polyacrylonitrile fiber（PANF） and polyester fiber（PETF） on flexural strength of aluminate cement modified by ASS were studied. The CO2 resistant performance of aluminate cement modified by ASS with 03% carbon fiber added（aluminate cement modified by ASSC） was tested and analyzed. The results show that the combined admixture ASS has synergist effect on the compressive strength of aluminate cement, resulting the reduction of compressive strength at late period. Carbon fiber can enhance the flexural strength without adverse impact on compressive strength of aluminate cement modified by ASS. The aluminate cement modified by ASSC has outstanding CO2 resistant performance, and its mechanical property doesnt decline after curing at high temperature and high pCO2 for 50d.