The invention discloses an assembling block adjuster and a construction method for unsupported air assembling of steel bridges. The assembling block adjuster comprises a U-shaped adjusting groove, a jack and a jacking support. The construction method includes the steps of welding of hoisting rings of steel bridge assembling blocks, trial hoisting and hoisting of the steel bridge assembling blocks,fine adjustment of the hoisted steel bridge assembling blocks prior to welding and removal of hoisting equipment. The assembling block adjuster can be used for completing air welding and assembling of the bridges without tools such as strong backs, horizontal trestles and vertical trestles, so that the assembling seam width and the elevation among the steel bridge assembling blocks can be adjusted more conveniently, efficiently and precisely, and construction is more time efficient and cost saving. The construction method is capable of avoiding failure to set up temporary support frames in areas with complicated conditions and can omit the process steps of support frame set-up foundation reinforcement, foundation bearing capacity detection, setting up of the support frames, support framejacking, support frame dismantlement and recovery of bottom surfaces of a support frame foundation, so that construction is accelerated substantially with lower construction cost.