The utility model discloses an on-line monitoring device and system for secondary pressure drop/secondary load. The on-line monitoring device for the secondary pressure drop/secondary load comprises a microprocessor, a first communication unit, a multiplexing selection unit, a signal conditioning unit, a first frequency multiplication unit, an analog-digital conversion unit, a second communication unit and a keyboard input unit, wherein the microprocessor is connected with the signal conditioning unit, the multiplexing selection unit, the first frequency multiplication unit, the analog-digital conversion unit, the second communication unit, the keyboard input unit and the first communication unit respectively. The on-line monitoring device and system provided by the utility model can on-line monitor main parameters such as load and error of a metering device in real time; power loss problem possibly caused by conventional regular monitoring can be prevented; in-site measurement can be carried out without labor; workload is reduced; work efficiency is increased; relevancy and practicability of measuring error are enhanced; and the problems in the conventional labor in-site measurement, such as large workload, low efficiency, time-sharing measurement for various errors, isolation and independence, and incapability for measuring comprehensive error of the power metering device in real time, can be overcome.