The spiral steel stair is widespread used as indoor stair for its graceful appearance. For the space structure system of the spiral stair, it is necessary to make vibration analysis and comfort evaluation to the stair. The load model of pedestrian and the analysis condition by walking and running in the process of ascending and descending are presented in this paper. Referring to some design code and research achievement of footbridge and floor, the index of comfort evaluation, which are the natural frequency, the peak vertical acceleration and the weighted root-mean-square (r.m.s) acceleration, are put forward in this paper. Finally, the numerical analysis and the vibration site test of a practical project are made and the results of vibration analysis and comfortability evaluation are presented to demonstrate the suggestion by the authors.