The stair is the vertical transport hinge among stories, it becomes a very important component. the stair is seriously damaged in Wenchuan earthquake, and it failed to perform the role for person escaping in case of emergency. a reinforced concrete frame structure with plate stairs as an example, the finite analysis is used. Calculation result shows that plate stairs have a great influence on earthquake resistant behavior of the structure. Designing without the plate staircase in consideration would lead to structural weakness in some columns and cause seriously damage under earthquake and affect the safety of structure.