The invention relates to an online detection device and method for a proportioning belt scale, and belongs to the field of weighing apparatus measurement. The online detection device and method are mainly used for solving a problem of carrying out online detecting accuracy of the proportioning belt scale in the production process. By adding the online detection device, accuracy of the proportioning belt scale is verified; and by calculating a detected weight, correction or early warning is carried out on an error of the proportioning belt scale. The online detection device and method can implement real-time monitoring on the proportioning error of the proportioning belt scale in the production process and timely corrects the proportioning scale so as to ensure quality of a mixed material.