Abstract: Using eyes with high concentration for long periods can cause visual fatigue. With the continuous development and progression of modern electronic devices, screens have become an integral part of many aspects of life. Watching a screen for a long time can cause extreme eye fatigue and accidents. A wireless monitoring system including multiple communication platforms is a new means of monitoring in confined spaces, which requires people to perform visual display terminal (VDT) operations in common operating places such as dispatch rooms, cabins, and shipyards. Visual fatigue in a confined space is one of the main causes of accidents. To explore the effect of lighting in a limited space on visual fatigue of VDT, 24 operators were selected to perform VDT typing in a confined space platform for 1 h, and seven light gradients were set within the range of 50–700 lx to collect pupil diameter data using an eye tracker. The collected data were normalized to reduce noise. Experimental results show that with an increase in illuminance, the pupil diameter generally decreases and the pupil–illuminance relationship conforms to the power function relationship. In high-illumination environments (400, 550, and 700 lx), the pupil diameter change rate fluctuates in the −12%–8% range, and with the increase in light intensity, the degree of visual fatigue of workers increases. Under low-illumination environments (50, 100, and 200 lx), the pupil diameter change rate fluctuates in the range of −8%−4%, and the degree of visual fatigue of the workers also increases with decreased intensity. This study proposes to use the windowed pupil diameter standard deviation, σ, to determine the time of visual fatigue. The peak value of σ under low illumination is earlier than that under high illumination; the peak value of σ under 300-lx illumination is the latest, weak illumination. The fatigue degree of vision caused by 50−300 lx is greater than that caused by strong illumination of 300−700 lx.