Monitoring and measurement systems for railway subgrade settlement using a chain of cameras can realize high-precision, non-contact railway subgrade settlement monitoring with the image measurement method. The precision and sensitivity of image measurement requires accurate and rapid evaluation of system performance, and regular maintenance should be changed to dynamic maintenance. Herein we propose a strategy for determining system performance using the spot-image quality and introduce four parameters, viz. edge definition, relative brightness, image information entropy, and light vertical degree, to evaluate the quality of light spots. The weight of each parameter can be constructed using the analytic hierarchy process and the entropy method. A comprehensive evaluation model for spot quality is constructed. Simulation and field experiments are conducted. The simulation shows that the proposed model can track changes in the spot-image quality score with good sensitivity. The field experiment shows that this model can be used to accurately evaluate spot quality. In addition, when the quality score is set at 0.95, the positioning error exceeds 1 mm (the allowable error range in subsidence for settlement measurement). The system requires maintenance when the light spot-image quality score continuously misses the threshold and exceeds the resting time. This method realizes system-state real-time monitoring and dynamic maintenance.