Global Navigation Satellite Systems(GNSS) have been widely applied in mass consumer applications, mechanical control, maritime and aviation navigation, and disaster relief. However, due to the inherent weakness of navigation signals and the open user interface of GNSS, GNSS is highly vulnerable to spoofing attacks. Signal Quality Monitoring(SQM) technology offers a simple yet proven approach for detecting GNSS spoofing attacks. Nevertheless, traditional SQM algorithms are vulnerable to environmental factors, making it difficult to simultaneously satisfy the basic requirements of detection accuracy and timeliness. This paper proposes a spoofing detection algorithm based on the sum of absolute values from the in-phase(I) and quadrature(Q) branches. The implementation logic is simple and requires only minor software modifications to GNSS receivers. Experimental analysis using the spoofing signal dataset from the University of Texas at Austin(TEXBAT) demonstrates that, compared with conventional SQM algorithms, the proposed algorithm achieves faster detection speed and superior performance in detecting spoofing attacks.