Carbon fiber is a kind of optimal materials for bridge maintenance and reinforcement. Design method of strengthening steel-concrete composite bridges using carbon fiber was studied in this paper. Characteristic of bridge reinforcement under load and phasing of stress features were considered, and calculation method of ultimate limit state was suitable for existing bridge design code . Ultimate strain of carbon fiber in ultimate limit state was firstly established based on plane-section assumption and strain-lag of carbon fiber. Then computational scheme and calculation method of flexural capacity in positive moment area and negative moment area for strengthening steel-concrete composite beam using carbon fiber were established respectively, which was reference for design of bridge strengthening.