Abstract: The spindle of a CSP mill was researched as continuous distributed mass. The spindle's angle, self-weight, mass unbalance and damp were all considered as influential factors. The equations of lateral-torsional coupling vibration for the spindle were established based on the dynamical theory to theoretically interpret field testing data. According to the differential equation of the mathematic model, the following conclusions can be drawn:if unbalance exists, the lateral vibration and torsional vibration are coupled distinctly; if unbalance does not exist, the torsional vibration can affect the lateral vibration, but the lateral vibration has not any effect on the torsional vibration.