The necessity of monitoring the transformer neutral line direct current in the hybrid AC/DC systems is elaborated. The integrated process from substation data acquisition to data issue is introduced. The design of direct current gathering installation is based on Hall theory and the magnetism principle of detailed balance. The various function modules of the monitoring system are mainly constructed based on the soft bus and the opening database connection. The effectiveness of the monitoring system is finally explained by an example， which is described with a curve and a group of numerals.