Abstract: In large-scale crude oil tankfarms, the supervisory control and data acquisition system (SCADA) is inefficient in data statistics and prone to downtime problem frequently due to calculation. In this regard, a new inventory monitoring system for crude oil tankfarms is proposed by combing OPC technical standard and MODBUS industrial bus together. With this new system, tank level data collected by MODBUS bus is directly displayed by SCADA, without data statistics and calculation. All the operating data of crude oil tanks are synchronously acquired by the metering software TankMaster and then sent to SCADA after complex background data calculation. It effectively combines the advantages of SCADA and TankMaster, so as to realize multiple redundancy of data communication, processing and display and reduce calculation load of SCADA greatly. In this way, data acquisition is more reliable and stable. The system provides a new way to improve the data calculation capacity of SCADA.