The invention relates to an online detecting device of coal components, wherein the online detecting device and a conveyor belt of a coal sample are installed together. The detecting device provided by the invention comprises an electrically-controllable deuterium neutron generator, a graphite reflecting body, a moderating body, a boracic polyethylene protective block, a lanthanum bromide detector, a temperature control system, a lead absorption sleeve, a shielding protective body, an amplifier, a multi-channel analyzer, a data processing system, an industrial personal computer, a data display system and a remote login system, wherein the neutrons which are generated by the small-sized electrically-controllable deuterium neutron generator with 2.5MeV are irradiated to coal streams which pass through the conveyor belt; the neutrons are in nuclear reaction with the main nuclide in the coal streams to generate characteristic gamma rays; and all the main nuclide in the coal can be detected in an online mode according to the properties of the characteristic gamma rays, thereby analyzing the water, ash components and heat values in the coal. The online detecting device provided by the invention is not influenced by the coal type and the dimension of granules and can carry out the real-time online detection on the element components in all the coal steams and industrial characteristic indexes.