Based on the strength of existing material, the research studies the ultimate span of steel cable and CFRP cable suspension bridge using analytic method. The result shows that the diameter of steel cable will increase sharply and its self-weight will account for 95% of total loads when the span is over 4 500m, which is no more suitable to adopt; however, the area of CFRP cable can still grow steadily when the span is over 5 000 m and its value is far less than the one of steel cable in same span, which is rather feasible. In addition, steel cable and CFRP cable suspension bridge with main span of 1 500, 2 000, 3 000, 4 000, 5 000m are tried to be designed and comparative analysis of their mechanical properties are made in the study. In conclusion, it is suitable to use steel cable when the span is less than 2 000 m while CFRP cable is more feasible when it is over 3 000m.