Simultaneous MRSI and T2* mapping has been demonstrated recently using SPICE. With a T2 map, T2' values could be calculated, which reflects tissue oxygenation. However, the accuracy of T2* and T2 measurements suffers from system imperfections. In this work, we improved T2' mapping by overcoming signal dephasing in T2* mapping and estimation bias in T2 mapping. The signal dephasing in T2* caused by B0 inhomogeneity was corrected utilizing high-resolution field map and pre-learned subspaces, and the estimation bias in T2 caused by B1+ inhomogeneity was corrected with a dictionary-based estimation. The proposed method provided improved T2' mapping in SPICE experiments.