We present a systematic investigation on the enhancement of the quality (Q) factors for guided-mode resonance (GMR) sensors with shallow subwavelength grating structures. By introducing the coupled-mode model, a theoretical high-Q factor can be achieved by choosing the proper geometric structure. Based on this method, a GMR sensor with a Q factor up to 8000, which is an order of magnitude larger than those of typical GMR sensors with Q factors within 100 ∼ 300, was demonstrated experimentally. Besides, the approached GMR sensor achieved a bulk sensitivity of 135 nm RIU-1 with a high signal to noise ratio, which supports a detection limit of 1 ng ml-1 for bovine serum albumin detection. This high performance GMR sensor paves the way towards high-throughput industrial mass production, and shows great potential for other applications, such as optical filters, spectrometer, and bio-imaging.