We propose an on-chip optical narrowband reflector (NBR) based on two cascaded Bragg gratings (BGs). A π phase shifted anti-symmetric Bragg grating (π-PS-ASBG) and a rear uniform Bragg grating (UBG), are in-line connected. The π-PS-ASBG provides a hybrid mode resonance between the even- and odd TE (TE<sub>0</sub> and TE<sub>1</sub>) modes, while the UBG is used as a rear reflector to reflect the TE<sub>0</sub> mode that transmitted from the π-PS-ASBG. The reflection bandwidth decreases when the coupling coefficient increases, which is different from traditional UBG. The performances are theoretically studied via both the rigorous three-dimensional finite-difference time-domain (3D-FDTD) method and multimode transmission matrix method (MTMM). The calculated 3-dB bandwidth is 0.16 nm when the whole grating length is 400 μm (π-PS-ASBG: 200 μm and UBG: 200 μm) and the coupling coefficients of the π-PS-ASBG and rear UBG are 240cm<sup>-1</sup> and 110cm<sup>-1</sup>, respectively. The proposed NBR can be applied in the cases requiring narrow reflection such as narrow linewidth semiconductor lasers.