Abstract: In this paper, fifty subjects were selected to complete three different types of continuous long-term mental tasks, and subjective self-reporting measures, behavioral performance and P300 components of event-related potential (ERP) were used to analyze the fatigue state before and after the experiment. The results showed that after a long-term mental task, the subjective sensory fatigue increased, the average response time of the complex arithmetic task increased significantly, the error rate of the switch task increased significantly, and the amplitude of P300 at the Fz, Cz and Pz electrode positions decreased significantly, indicating that the ability of the brain's information processing, emergency response and judge decline. The results also verified that three types of mental tasks have different degrees to induce the increase of mental fatigue. In order to make full use of multi-channel electroencephalogram (EEG)'s information, the linear descriptor parameters were put forward to analyze the change of brain's overall state before and after the experimental task. The study results showed that the energy of the central and parietal brain regions increased after the task was completed, while the changes of the field strength in the forehead, central and parietal lobe decreased. The linear descriptor parameters can comprehensively describe the physical characteristics of the changes in the overall state of the brain's activity caused by the change of the mental fatigue depth from different angles, so as to better reveal the mechanism of mental fatigue. It opens a new window for multi-channel EEG's analysis of mental fatigue.