The complex structural behaviors in steel spiral staircases， due to their unique and irregular structures， render traditional linear analysis methods inadequate for accurate prediction of structural responses and stability design， particularly in determining effective lengths. This study employs the direct analysis method （DAM） to overcome these limitations， taking into account the overall structural and member initial imperfections， thus providing an efficient and safe approach for the analysis and design of such structures. A specific engineering project is presented as a case study， utilizing NIDA software to perform a stability investigation via DAM， with results compared against those obtained through linear analysis. The research examines the mechanical responses under full-span loading， left and right half-span loading， and various live load distributions across different sectors， as well as the impact of varying support conditions on the staircase structure. The findings indicate that the maximum component stress determined by DAM exceeds that of first-order linear analysis， with a maximum utilization factor reaching 0.993， and a corresponding vertical displacement that is even more significant， peaking at 0.348 m， confirming the importance of second-order effects. In the design of steel spiral staircases， adverse distributions of live loads， especially under left half-span loading conditions， should be given special consideration. For live load distributions， the impact is greatest in sectors 3 and 4， located at the mid-span of the staircase. Moreover， increasing the stiffness of the top support contributes to a reduced stress ratio while also leading to an increase in bending moments due to second-order effects. This research plays a significant role in guiding the analysis and design of steel spiral staircases.