Visual fatigue cannot only lead to decreased productivity and vision, it might also bring various accidental injuries. How to detect visual fatigue in a simple and fast way is an urgent topic. This paper proposes a visual fatigue detection approach with the aid of special characters searching and reading tasks. Participants are required to locate these special characters from the reading materials through scanning, and at the same time complete the corresponding reading comprehension tasks. The experiment is conducted under the monitoring of Tobii X230 eye tracker. Eye tracking data, such as pupil diameter ratio, number of AOIs etc. are recorded. BP neural network is employed to determine the state of visual fatigue. The experimental results on 10 users show a classification accuracy of 95.2%, which confirms the effectiveness of the approach.