In recent years, miniature spectrometers have been found useful in many applications to resolve spectrum signature of objects or materials. In this paper, algorithms for filter-array spectrum sensor to realize miniature spectrometers are investigated. Conventionally, the filter-array spectrum sensor can be modeled as an over-determined problem, and the spectrum can be reconstructed by solving a set of linear equations. On the contrary, we model the spectrum reconstruction process as an under-determined problem, and bring up the concept of template-selection by sparse representation. A proposed heuristic algorithm is tested to achieve a high reconstruction resolution. Simulation results show that superior quality of spectrum reconstruction can be achieved from this under-determined approach.