Research conclusions: Taking construction of the underneath railway frame bridge at Lvshun North Road in Dalian as an example，this paper analyzes the load beating capacity of the reinforcement system for track on I—beam．The reinforcement system with longitudinal beam as the primary load—beating element can realize the unsupported jacking，but can only be applied in some small—span frame bridges，while the reinforcement system with the crossbeam as the primary load—beating element are more suitable for the large—span jack—in frame bridges．Besides，this paper offers a new way of calculating the beating capacity of the reinforcement system as the space structure．This method is easy， accurate and effective and it is wonh being applied widely．