The invention provides a risk-based short-term capacity expansion decision-making method for an oil-immersed transformer. The method comprises the following steps of calculating a hot-spot temperature change situation of the transformer by utilizing load guidelines of the oil-immersed transformer: calculating the fault rate and economic profit and loss of the transformer based on the hot-spot temperature change; assessing probability distribution of a hot-spot temperature according to the prediction of a load rate and an environment temperature, and determining a capacity expansion risk; and making a decision for a capacity expansion plan according to a calculation result. Therefore, the transmission and power supply capacity of existing power transmission and transformation equipment is improved; the power supply and demand contradiction is relieved; the resources are saved; and the economic benefits of power enterprises are increased. The method belongs to the technical field of transformer capacity expansion plan making.