The invention provides an alternating current (AC) charge induction type pulverized coal fineness online monitoring method and an AC charge induction type pulverized coal fineness online monitoring device; a bran-new pulverized coal fineness online measuring device is provided by utilizing an AC charge induction technology, a digital signal processing technique and a correlation principle. The device comprises a pair of charge induction type sensors which are arranged on a pipeline, are parallel to each other and are vertical to the pipeline; probes of the charge induction type sensors are arranged in the pipeline; connectors of the charge induction type sensors are arranged outside the pipeline and are respectively connected with two input ends of a signal processing unit. The bran-new pulverized coal fineness online measuring device can be used for directly measuring the relative change value of the pulverized coal fineness in an online way; the online measurement of the absolute value of the pulverized coal fineness can be realized by manual calibration; the device is not influenced by the parameter variation of medium working condition, and has the characteristic of being maintenance-free.