The invention discloses a coal mill pulverized coal fineness online monitoring method; according to a pre-established model, during running, the coal mill dynamic separator revolving speed, the coal mill inlet primary air volume, the coal feeder coal-feeding quantity, the coal mill current, the coal Hardgrove grindability index and other parameters are acquired through a DCS system for estimating the fineness of current pulverized coal, and the reference is provided for the operation of a pulverizing system; the online monitoring method is used in the pulverized coal furnace pulverizing system, the outlet pulverized coal fineness can be monitored, and the economy is better compared with that of a system without monitoring the fineness; the method only utilizes DCS data, fire coal daily records and other related records in existing operation data, has no need of special sampling and measuring devices, is simple and practical, and avoids the problems of abrasion and blocking of sampling and measuring devices in an existing pulverized coal fineness physical measurement method.