Based on the consideration of economy and being easy to construct on site, Cr5Mo/A302 dissimilar welded joints are widely used in elevated temperature applications such as coal-fired power station, nuclear plant and petrochemical industry. Because of the difference in carbide forming elements on both sides of welded joint fusion line, carbon diffusion will happen in service progress and induce premature invalidation. The calculation methods of carbon diffusion and its harm to creep life of dissimilar welded joints have been investigated in the past decades. Theoretically, at a certain temperature, creep damage mechanism changes according to stress levels. However, the previous works paid little attention on the effect of carbon diffusion to creep life at different stress levels and few studies have been done focusing on relationships among carbon diffusion degree, stress level and creep life. In this work, carbon diffusion behavior of Cr5Mo/A302 dissimilar welded joint was first studied by employing aging treatment test, micro-hardness measurement and Fick's second law. Subsequently, creep tests were performed to investigate the effect of 200 mm wide decarburized zone on the joint creep life at different stress levels. The results showed that carbon diffusion in Cr5Mo/A302 dissimilar welded joints became serious with the increase of aging treatment time. The widths of carbon rich zone and decarburized zone were both consistent with the parabola distribution law and could be simulated by Fick's second law. The decarburised zone, which formed in the process of aging, reduced the creep life of the joint greatly at high stress levels. However, with the decline of testing load, its effect became much smaller. When the stress level dropped to about 36% of the yield stress, its effect was negligible. Meanwhile, the relationship of stress and carbon diffusion impact factor S was established to determine the critical value of the stress below which the decarburised zone would not affect the high-temperature creep life of the dissimilar joint.