This paper addresses the design method of spatial filter for suppressing modal observation spillover. To eliminate the several selected modal amplitudes from structural vibration responses, a kind of modal filter is introduced. This filtering methodology can be realized by a small number of sensors, relative to the number of dominant modes in target frequency range. Proposed filter may be designed by using sensor placement to realize that the two subspaces, which are respectively spanned by a spatial filter vector and the selected target modal vectors, are orthocomplementary. Then observation spillover associated with these selected modes is suppressed. Effectiveness of proposed filter is verified numerically for a cantilever beam. Simulation results have shown its robustness for measurement error and estimation error in modal vectors, and its potential in suppressing the spillover in vibration control and health monitoring.