An antenna array system with high angular resolution is proposed to adapt the demands of both medium-range radar (MRR) and long-range radar (LRR) detections for 77 GHz automotive radars. Both the MRR and LRR modes are integrated into one substrate based on the optimized sparse array topology, which makes full use of the antenna aperture size to improve the angular resolution of the proposed system. Two-dimensional series-fed weighting arrays are designed via the Taylor synthesis method to effectively heighten the antenna gain and restrain the sidelobe level. After completing the fabrication, measurement results of the proposed antenna array are in good agreement with the simulation results. Moreover, the angular resolution is verified to be 0.5 by adopting the coherent signal space method (CSM) with stepped frequency transmitting waveform, which validates the effectiveness of the proposal.