The invention provides a construction method for a viaduct in a narrow space. The construction method for the viaduct in the narrow space comprises the following steps that 1, a
pile foundation platform is constructed; 2, constructing a cast-in-place bent cap; step 3, mounting a portal structure; step 4, component acceptance and transportation; 5, on-site assembling is conducted; step 6, mounting a temporary bracket; step 7, trial hoisting; 8, mounting the
box girder; ninthly, the support is detached; the portal structure is slidably mounted on the rail distribution beam arranged on the non-
motorized vehicle lane, the temporary support foundation is arranged on the traffic lane, so that the temporary support disperses load through the column bottom distribution beam, and the top is provided with the adjusting steel
pipe to finely adjust the elevation of the box beam. The box girders are spliced in sections, the weight of the portal structure hoisted at a time can be reduced through sectional
welding forming of the connecting cross beams, the portal structure adapts to small-space operation, and accumulative errors are reduced through sectional
welding; a clearance is reserved during disassembly, collision between the
box girder and the support is avoided, the disassembly process is controllable, and the integrity of a main body structure is protected.