This paper uses VB to design the monitoring System of Coagulant Dosage based on the software of decision-making aided system of water treatment development project. The monitoring System of Coagulant Dosage finishes the data acquisition, display and save, curve, alarm, control and other functions. The monitoring System of Coagulant Dosage is in correspondence with control device. The running results show that the system can meet the demands of monitor.