Abstract: The mill building for old assembling workshop for modern automobiles in Beijing is the one by reforming an old factory building established in 1980s.In order to save land and investment the principle of new technique and old mill building was combined,i.e.the construction was done while the production was going on. This old workshop was the building of triple-span(18+18+24)m.It was required by the new technique that the roof truss should be lifted by 5.5m within 24m span(60m24m) of three spans.The design of strengthening and lifting the mill building was done,thus integrating new space with the old one,which could meet the service requirements.