Bit condition affects weather straightener can bite and straighten rolled pieces fluently. In the paper, a mechanics model in plate straightening process was established, and bit condition formula was deduced. Considering advance contact phenomenon, a mechanics model in stable straightening process was given. According to section steals straightening process, the analysis indicated that bite condition for section steals was same as plate. The method to get section steals straightener’s best installation height for was proposed. The application showed that models in the paper were true, and could be guide for product.