A new theory of narrow bandwidth photonic crystal optical filter, which consists of GaAs/AlxGa1－xAs multiple quantum wells, is proposed. The anomalous dispersion behavior caused by excitons absorption in GaAs/AlxGa1－xAs MQW under an external electric field is investigated. The results show that there is a strong dispersion and small absorption region in the near resonant frequency of the anomalous dispersion material. Thus, a narrow transmission band will appear in the photonic band gap due to the dispersion in that region, of which corresponding maximum transmission value is about 0.73 and the bandwidth of the all solidstate filter is about 4.9 nm. This filter can be tuned by an external electric field. It provides guidance for the design of the new solidstate filter.