Abstract: This study is to investigate the demand mechanism of pounding required separation length of high-pier railway bridges at different site soil conditions and also to provide the evaluation and the computing theory method of pounding required separation distance respectively for existing bridges and in-built bridges. Firstly, the spatial seismic model considering multiple supported excitations was developed and the mathematical formulation was derived about the relation of varied local site soil conditions and acceleration at the bottom of proposed bridge piers; Secondly, the finite element model of a high-pier railway bridge was developed, and the relation of the separation distance between the adjacent bridge decks and the site soil condition at various earthquake levels was obtained; Finally, a real high-pier railway bridge was employed to analyze the require length of adjacent decks due to different local site soil conditions and the diverse earthquake magnitudes. The comparison analysis of the relative displacement responses at the uniform site soil condition and the practical site soil distribution was performed. The study indicates that the peak value demand of power spectral density of required separation length according to uniform soft soil distribution is the maximum, and 4~6 times as large as that in firm site condition distribution. The phase of power spectral density function of the required separation length varies largely and the number of deck pounding increases at real site soil condition compared to the uniform site distribution. At different earthquake levels, the demand mean values of pounding required separation distance in real site soil condition distribution are the greatest, and are 76%, 71%, and 42% respectively greater compared to firm, medium, and soft site of uniform distributions.