The invention relates to an ultra-smooth plane grinding polishing device and method for online trimming a flexible polishing pad. The ultra-smooth plane grinding polishing device comprises a cluster magneto-rheological abrasive semi-fixed flexible polishing pad generating device, a workpiece clamping and intermittent adjusting mechanism and a flexible polishing pad real-time trimming device, wherein the cluster magneto-rheological abrasive semi-fixed flexible polishing pad generating device comprises an annular grinding disc, a cluster magnetic pole, a separation disc, a lower end cover and a rotating plate, the workpiece clamping and intermittent adjusting mechanism comprises an inner supporting ring, an outer supporting ring, a planetary gear, a movable cylinder, an upper end cover, a depth scale, a connection shaft, an expansion sleeve, an inner gear and an outer gear, and the flexible polishing pad real-time online trimming device comprises a trimming planetary gear and a magnetic pole. By adopting the ultra-smooth plane grinding polishing device, the whole process of grinding, coarse polishing and fine polishing of the workpiece under the situation that the magneto-rheological liquid is not replaced can be realized, the surface quality of the workpiece is good, and the machining efficiency is high; moreover, the surface and subsurface damage can be avoided, the cost is low, and the ultra-smooth plane grinding polishing device can be used for machining different optical elements and semiconductor substrates.