The invention relates to a transformer substation control unit-based method for calculating increasable and decreasable reactive power of main transformer low-voltage sides, and belongs to the technical field of automatic voltage control of a power system. The method comprises the following steps: firstly, forming a control unit of a transformer substation according to the topological structure of a current power grid; calculating increasable and decreasable reactive power of each main transformer low-voltage side according to the state of reactive power equipment in the control unit; and during calculation, firstly considering the reactive power equipment with capability, eliminating the reactive power equipment which cannot be input or cut off due to the constraints such as safety rules, operation rules and voltage security requirements of operation equipment according to the requirements, and finally counting the reactive power equipment which can be calculated to determine the increasable and decreasable reactive power of the main transformer low-voltage sides of the control unit of the transformer substation. According to the method disclosed by the invention, statistical calculation can be rapidly and correctly carried out according to the increasable and decreasable reactive power of the main transformer low-voltage sides according to the control unit of the transformer substation; the constraint for optimal computation of the reactive power of the whole power grid is obtained; and the requirements of automatic voltage control of the transformer substation are met.