A dynamic spectrum sharing method based on user online learning and low-overhead cooperation is provided. In the method, a secondary user (SU) obtains spectrum sharing scene information and requirement parameters; then initializes state parameters and spectrum sensing parameters of a channel. At the beginning of each slot, the SU determines whether a direct transmission condition is met according to the transmission condition of the previous slot, and directly continues transmission on the channel of the previous slot or obtains an idle channel according to the channel state and spectrum sensing for transmission, and updates state parameter and spectrum sensing parameter. Finally, an SU base station performs conflict coordination according to the channel occupancy condition for a fixed number of slots. Hence, low-overhead and efficient spectrum sharing is realized by the SU learning the channel state and the transmission condition, such that the spectrum utilization rate in a high user density and dynamic spectrum environment can be effectively improved.