The invention discloses a double track bridge construction process crossing existing railway operational lines, and the constructed double track bridge comprises a front side bridge section, a crossed existing line bridge section with mutually connected front and rear ends and a rear side bridge section. The process comprises the following steps: 1) synchronously constructing the front side bridge section, the crossed existing line bridge section and the rear side bridge section, wherein the crossed existing line bridge section is constructed as follows: binding reinforcement cages, concreting, assembling temporary buttresses and D-shaped temporary beams, arranging movable slide rails, carrying out translation insertion on the temporary buttresses, setting up temporary beam supporters, hoisting the temporary beams, installing cap beam forming templates, concreting cap beams and removing all temporary strut members; and 2) folding the bridge sections. The invention has the advantages of reasonable design, convenient construction, low investment cost, good use effect, high construction efficiency, short construction period and low safety risk, and can effectively solve the actual problems such as large construction difficulty, long construction period, high investment cost, high safety risk and the like in the construction process of the crossed existing line gate piers.