The chemical oxygen demand (COD) online monitoring sensor, based on the spectral absorption, shows outstanding advantages in water quality monitoring, pollution source tracing, and environmental protection inspection. However, there are several factors that influence the stability and accuracy of the sensor detection in practical application, and?therefore,?they?must be?carefully considered. This research analyzes the impact of changes in physical characteristics, chemical composition, and external environmental interference on the output result of the optical COD sensor systematically and comprehensively, from different fields including electronic technology, sensor technology, composition analysis, and environmental condition analysis, through both theoretical and experimental approaches. This study will provide guidance for the scientific research, product development, and upgrading of the optical water quality sensor, and provide reference for the long-term use, operation, and maintenance of the sensor.