The multichip SiC power module is a promising alternative of the Si counterpart for the power control unit of the electric vehicle. The thermal coupling effect among the multiple parallel SiC chips may pose a negative impact on the accuracy of transient thermal measurement, which is critical to the lifetime assessment and reliability of the SiC power module. In this paper, general mechanism models of the thermal impedance are created to reveal the layout-dominated relative error of the thermal impedance for the multichip SiC power module. Besides, to evaluate the influence of the layout on the measurement error of the thermal impedance, comparative evaluation of twelve power modules with the standard HybridPack packaging is performed based on Monte Carlo method. It is found that the maximum relative error of the thermal impedance is up to 75%. Based on a two-chip SiC power module, the equivalent electric and thermal circuits are built to illustrate the mechanism of the thermal coupling and chip parameter dispersion on the thermal impedance measurement. In addition, two four-chip SiC power modules with different layouts are fabricated to evaluate the relative error in the transient thermal measurement. Note that, in order to obtain appropriate accuracy of the transient thermal impedance, the advanced non-destructive method is further explored for the thermal assessment of the multichip SiC power module.