The invention relates to a direct-current transmission converter valve cooling water quality online monitoring system and method. The direct-current transmission converter valve cooling water quality online monitoring system and method are mainly used for monitoring water quality in a direct-current transmission converter station and related fields thereof to avoid the problem of scale formation of a cooling tower due to low water quality. A water sample to be detected is delivered to a special water quality analysis groove through a sample delivering pump, water quality is detected through instruments such as a thermometer, a conductivity meter, a hydrotimeter and a total alkalinity meter, results are transmitted to a data acquisition analyzer, and are calculated by built-in data analysis software, and states of the water sample are locally displayed or submitted to an upper computer. The system is easy to operate, visual in detection result and high in adaptability to detection environments, not only can be used for static detection, but also can be used for real-time online monitoring. By the method, common workers can rapidly take measures to solve problems conveniently.