In recent years, research on transformer insulation aging evaluation via methanol in oil has advanced. However, practical applications show oil flow induces heterogeneous methanol distribution inside the transformer tank, causing evaluation errors and limiting the method’s promotion. This paper investigates methanol’s distribution characteristics and spatiotemporal variation in transformer oil. A finite element simulation model of the transformer is established, with internal parameters and simulation conditions clarified. Simulations analyze methanol’s distribution and spatiotemporal variation under different oil flows. Results indicate that under natural oil circulation, methanol mainly accumulates near windings, with slightly higher concentration in the upper tank than the lower; as forced oil circulation speed increases, methanol distribution tends to be uniform (smallest overall concentration difference at high speeds), though local temporary accumulation may occur due to complex flow fields. This study enhances the applicability and accuracy of the oil-paper insulation aging evaluation method based on methanol in oil.