A method for calculating a temperature-dependent mid-span vertical displacement of a girder bridge includes: setting a joint rotation of a main girder at each support as an unknown quantity, and establishing an equation according to a bending moment equilibrium condition at the joint; then introducing a sequence to establish a quantitative relationship between each unknown quantity; substituting the relationship into the equation, to obtain an analytical formula for a rotation at each joint; establishing an analytical formula for a bending moment at each joint through a principle of superposition; and finally, establishing an analytical formula for a mid-span vertical displacement of each span girder through a principle of virtual work. This method provides an analytical formula with exact solutions for prismatic girder bridges which have equal side spans yet have any number of spans.