The invention disclose a setting calculation method for a bus differential protection current loop line disconnection alarm fixed value, which solves the problem that the current loop line disconnection alarm signal of the transformer substation bus protection frequently happens due to the fact that the current transformer substation bus differential protection current loop line disconnection alarm fixed value is not reasonable. The setting calculation method comprises steps of establishing an imbalance current calculation model of a power transmission circuit according to the usage tower types, the lead phase sequence arrangement and the line length of all power transmission circuits which are connected to one bus of the transformer substation, calculating the positive sequence current, the negative sequence current and the zero sequence current of each loop of the power transmission circuit and the imbalance degree of the zero sequence current, performing actual measurement on the secondary zero sequence imbalance current of all power transmission lines after the transformer substation is in operation, and performing adjustment on the bus protection current loop line disconnection alarm fixed value according to whether the degree of imbalance of the zero sequence current is not greater than 5%. The setting calculation method overcomes the frequent actions of the bus protection current loop line disconnection alarm signal of the transformer station, and guarantees the safety operation of the power grid.