Based on the wide-side coupled stripline structure, a 3 dB 90° bridge with high isolation and low insertion loss based on low-temperature co-fired ceramic (LTCC) technology is designed in this paper. The spiral coupling line is used to effectively reduce the size of the bridge, and the symmetrical structure modeling is more convenient for later optimization and adjustment. By introducing an extended adjustable isolation capacitor in the vertical directions of the wide-side spiral coupling stripline, the isolation of the bridge is greatly improved, making it up to 27 dB, and the insertion loss is less than or equal to 0.2 dB. Compared with the traditional directional coupler structure, this can greatly reduce the device size while improving the performance. The electric field intensity at the right-angle corner of the coupling line is analyzed and optimized, and the electric field intensity at the corner is roughly equal to that at the straight line by adopting the 45° oblique cutting method. At the same time, the upper grounding metal plate is hollowed out annularly, which improves the amplitude balance in the band. The designed 3 dB 90° bridge has passband of 0.96-1.53 GHz, insertion loss of ≤0.2 dB, amplitude balance of ≤±0.7 dB, phase balance of 90°±1°, and isolation of ≥27 dB, which has a good prospect application in market field.