In order to study the characteristics of the temperature variation in oil-immersed power transformer windings during operation, a two-dimensional transient fluid-thermal coupling calculation method for the transient temperature rising of transformer windings is proposed. Based on the dimensionless least-square finite element method (DLSFEM), the two-dimensional transient fluid-thermal coupling calculation method calculates the velocity distribution of the transformer flow field at different time instants, and based on the upwind finite element method, the temperature distribution at each moment is calculated. Considering the influence of the nonlinear material properties and winding Joule loss on the calculation results, the sequential iteration method is applied to solve the fluid-thermal coupling problem, and finally, the characteristics of the field temperature change are obtained. Compared with the traditional least-square finite element method, the DLSFEM has smaller stiffness matrix number conditions, and the corresponding discrete equations have better convergence. An oil-immersed power transformer winding model is taken as an instance, and the temperature distribution is calculated by the proposed method and the commercial computational fluid dynamic software Fluent. The calculation results of the proposed method are basically consistent with those of Fluent, and its iterative number is much less than that of Fluent, which greatly improves the calculation efficiency.