This paper based on the heat generation of iron core and windings of transformers, makes a detailed analysis of production costs of iron core and load losses and the influence on top oil temperature and hot-spot temperature of transformers. It also explains clearly the meanings of parameters in winding hot-point temperature equation recommended by GB/T 15164-1994 Loading Guide for Oil-immersed Power Transformers. And adjustments and revisions are made on the winding index and oil and fat item to get more reasonable corresponding physical significance of them. At the end, a comparison is made between the recommended equation calculation results and improved equation in loading guide. Although there is a similar tendency of hot-point temperature of the two and the calculation results are not in great different, a more reliability is in improved equation.