The two degrees of freedom (2DOF) mathematic model of grinding wheel shaft vibrating that arose from the unbalance relative to grinding carriage was established. The relationship between the unbalance of grinding wheel and vibrating displacement of spindle system was deduced. With theoretical analysis and experimental results, it shows that singles sampled with the mathematic model contain less interference by comparing with single degree of freedom vibrating mathematic model. The online detection precision of grinding wheel unbalance can be improved. At the same time, the state in working of grinding wheel shaft can be monitored in virtue of the sampled signal.