The invention relates to an online electric quantity compensating device which is mainly technically characterized by comprising a power circuit, an analog channel circuit, an A/D (Analog to Digital) conversion module, a counting processor circuit and a master controller circuit, wherein the power circuit is connected with the A/D conversion module, the counting processor circuit and the master controller circuit and used for supplying power to all the circuits; the analog channel circuit is connected with a junction box of an electric energy meter; and the analog channel circuit, the A/D conversion module, the counting processor circuit and the master controller circuit are sequentially connected together. The online electric quantity compensating device is reasonable in design and can be widely used for occasions requiring three-phase three-wire and three-phase four-wire electric energy meters with various voltage grades, which are provided with current transformers; the consumed meter electric quantity during meter replacing can be accurately recorded when a working staff replaces a meter on load without power outage; functions of browsing, inquiring, reporting and printing a test result are achieved; loss of electric quantity of enterprises is effectively avoided; and reasonable and impartial electric consumption transaction between electric power enterprises and electric consumption customers is ensured.