Based on the empirical formula recommended in GB/T 1094.7-2008 for the calculation of transient hot spot temperature with the index equation method, taking a S13-M-100 kVA/10 kV oil-immersed transformer as the research object, a correction factor is proposed for the situation where the empirical formula calculation error of transformer transient hot spot temperature is large when the empirical formula is applied to load reduction of the transformer and wind speed in the environment. Besides, the Levenberg Marquardt (L-M) algorithm is utilized to optimize the parameters of transformer thermal characteristics and wind speed correction factor, thus the calculation method of empirical formula transient hot spot temperature is improved. The hot spot temperature of a S13-M-100 kVA/10 kV transformer during the multi-stage temperature rise test is calculated with the improved empirical formula, and the maximum error between the calculation results and the test results is not more than 5 ℃, which verifies the effectiveness and reliability of the method proposed in this paper.