Abstract: Based on elastic-thin-plate bending theory, elastic-foundation theory and key strata theory in ground control, the mechanical mechanism of injecting grout into separated strata was discussed, and the mechanical model of the overlying strata and the evaluation model for the reducing-subsidence effectiveness by injecting grout into separated strata were established. A formula to calculate the deformation of key strata was derived from their flexing equations pre and post grouting. Combined with the concept of reducing-subsidence coefficient, a formula to evaluate the reducing-subsidence effectiveness by injecting grout into separated strata was obtained. The example shows that the theoretical result agrees well with practical observations and numerical simulations of injecting grout into separated strata.