The invention discloses a construction support and method for an overhead large-span heavy-duty cross beam. The construction support comprises a lower bracket supporting structure, an upper bracket supporting structure located above the lower bracket supporting structure, and a middle supporting structure with the bottom supported on the lower bracket supporting structure. The top face of the upper bracket supporting structure is flush with the top face of the middle supporting structure, and the upper bracket supporting structure and the middle supporting structure form a temporary supporting platform for supporting the cross beam to be constructed. A load bearing structure is erected on the temporary supporting platform. The construction method comprises the steps that firstly, the lower bracket supporting structure is installed; secondly, the upper bracket supporting structure and the middle supporting structure are installed; thirdly, the load bearing structure is erected; fourthly, the support is prepressed; fifthly, the upper cross beam is constructed; and sixthly, the support is removed. According to the construction support and method, design is reasonable, construction is easy and convenient, the using effect is good, and the problems that as for an existing floor steel pipe supporting system, cost is high, the erection process is complex, the construction period is long, the construction risk is high, the requirement for supporting stability is high, and the construction progress of the upper cross beam is affected can be effectively solved.