Hybrid integrated external cavity lasers (ECLs) using cascaded micro-rings with the advantage of wide tunable range and narrow linewidth, have rich applications. However, due to the limited bandwidth of the reflector, continuous correction of the reflectivity is required to sustain optimal ECL’s performance when spanning large wavelength ranges. Here, by introducing bent direction couplers in a Mach-Zehnder interferometer (MZI)-based reflector, we have designed a wide-bandwidth adjustable Sagnac-loop reflector (WASR) with a bandwidth over 100 nm. Moreover, the proposed external cavity was fabricated on the 800 nm-thick low loss Si3N4 waveguides. In experiment, we have measured a tuning range of over 81 nm and 90 nm for butt-coupling with two reflective semiconductor optical amplifiers (RSOAs) with different wavelength bands, and an intrinsic linewidth < 23.8 kHz and a side-mode suppression ratio (SMSR) > 50 dB across the entire tuning range were also obtained.