The utility model discloses an online flow measuring device for liquid with unstable flow, and belongs to the field of flow measuring devices. The online flow measuring device comprises a heat preservation anti-explosion device, a water storage device, a metering device, a controller, a valve, a valve transmission device, liquid level detection devices, a heating device, a display device, a communication interface and a mains supply input interface. One side of the online flow measuring device is connected with a 220V mains supply input, and the other side of the online flow measuring device is connected with an upper computer through the communication interface. A communication display device and a junction box 1 are fixed on the outer wall of the upper portion of the heat preservation anti-explosion device, the water storage device, the metering device and the controller are arranged in the heat preservation anti-explosion device which is provided with a liquid input connector and a liquid output connecter, and the liquid input connector and the liquid output connecter are connected with a pipeline of liquid to be measured respectively. Fluid flow is accurately measured and adjusted, so that a foundation is laid for safe and economical operation in the producing process, product quality improvement, material consumption reduction, economical benefit improvement and scientific management.