In this paper, the modified reference stress method is introduced to estimate the C* integral for collinear creep cracks near the mismatching bimaterial interface (MBI) and the process that leads to these solutions is also presented. The interacting factors for creep cracks near the MBI are defined and the influences of different creep exponents and mismatching factors (MF) on creep interacting effect are studied. Results show that if two inner creep crack tips get closer, the interacting effect of creep cracks near the MBI will become much stronger. Under the same condition, the interacting factors of the creep cracks in materials with higher creep exponent are larger than that of the creep cracks in materials with lower creep exponent. For the same crack location, C* integral decreases with the increase of MF. Two novel dimensionless parameters are proposed to characterize the rationality of combination rules of ASME, API 579, and R6 codes for the interacting effect for creep collinear cracks near the MBI. With the proposed parameter, the nonconservative ranges to use the combination rules of ASME, API 579, and R6 codes are rediscussed and presented.