Grating assisted coupler which consists of a mismatched coupler with a Bragg grating written in only one core over the coupling region is a kind of non-interference device, so it is stable in performance and doesn’t depend on the position of the grating in the coupler region.A coupler with uniform coupling region making of SMF-28 and photoconductive fiber is fabricated by improved fused taper technology,then a Bragg grating is written in the coupling region with 248 nm ultraviolet laser,and a narrow bandwidth grating assisted coupler with maximum about reflectivity 15 dB, transmission 4 dB and bandwidth 0.1 nm is achieved to satisfy the dense wavelength division multiplexing (DWDM) system; The effect of the uniformity of coupler on the response is studyed; Its filter performance is tested in the 10 Gb/s fiber optic transmission system, and the results show that the device has good performance.