Mismatch effect plays a crucial role in weldments, and an independent mismatch constraint parameter is proposed to characterize the material mismatch constraint effect in this paper. A mismatched modified boundary layer (MBL) model for creeping solids is developed to simulate the stress field of creep cracks in mismatched weldments. It can be found that there still exists the similarity between creep crack tip stress fields under different mismatch factors. Numerical results show that obtains the minimum value on the under match condition and the maximum value on the over match condition. Comparisons between and other geometric constraint parameters ( and ) are carried out and the applicability of is verified. A modified assessment formula for creep crack growth rate ratio is proposed based on the parameter . It is found that is a reasonable and remarkable parameter to characterize the mismatch constraint effect of creeping cracks.