[Objective] Research on an on-line detection method for total hardness of water quality to improve detection efficiency and ensure water quality in dairy factories. [Method] Install the ion electrode on the water pipeline leading to the dairy production workshop,equipped with water quality monitoring instruments,IoT boxes,collection platforms,water quality analysis platforms,computers,etc.,for real-time on-line detection and monitoring of water quality,automatic monitoring of water quality,real-time collection,and on-site warning notifications. [Result] Compared with the national standard method GB/T5750.4-2023 Standard Test Methods for Drinking Water Part 4：Sensory Characteristics and Physical Indicators,the on-line detection method saves labor,reagent and consumables costs,improves work efficiency. [Conclusion] This method achieves automatic real-time collection of total hardness of water quality,on-site warning notification,ensuring the efficiency,timeliness, accuracy,and sufficiency of system information,and has good application prospects..