Increased consumer preference for low carbon products provides many business opportunities; investment in reducing carbon emissions results in positive externalities in supply chains． This paper investigates the strategy of promoting investment in reducing carbon emissions for suppliers and manufacturers in a two-echelon supply chain under a contract with punishment mechanism． According to the different payoff matrices of suppliers and manufacturers when adopting different strategies，this paper develops an evolutionary game model，and proposes evolutionary stable strategies of investments in reducing carbon emissions for upstream and downstream firms． The results show that investment strategies of suppliers and manufacturers are related to the ratio of input-output． When the input-output ratios of both parties in supply chains change，some evolutionarily stable equilibrium is found． Finally，a numerical verification for the mathematical model is given． If a‘free rider’can gain a lot in a supply chain，suppliers or manufacturers will not invest in reducing carbon emissions．