The invention relates to a cereal moisture content online device based on dielectric parameters and a measurement method using the cereal moisture content online device. The cereal moisture content online device comprises a coaxial column-type dielectric parameter measurement sensor and a measure controller, wherein the coaxial column-type dielectric parameter measurement sensor comprises an inner electrode assembly, an outer electrode, a polytetrafluoroethylene insulating layer, a shielding layer and a polytetrafluoroethylene protecting layer which are sequentially and coaxially mounted in a multilayer nesting manner from inside to outside; and the measurement controller is electrically connected with the coaxial column-type dielectric parameter measurement sensor through a double-layer shielding type coaxial cable. According to the cereal moisture content online device, the moisture content of a flowing cereal in a coaxial column-type electrode accommodating cavity is calculated by measuring the dielectric constant epsilon', the dielectric loss epsilon'' and the temperature T of the cereal, and the measurement error caused by the bulk density fluctuation of the cereal in the electrode accommodating cavity is corrected in an application range by virtue of a bulk density correction formula (shown in the description), so that the measurement precision is improved, and the online real-time measurement of the cereal moisture content is realized.