Previous studies have shown that ambient illumination, screen brightness and contrast of the computer could affect the user's visual task performance. In the era of mobile internet, mobile are more and more important in people's work and life. Domestic and foreign researchers have conducted extensive research in ergonomics factors which could affect the display performance. These factors included the display object, the environmental illuminance, the screen brightness and the contrast, etc. Early studies were mostly concerned about CRT or large LCD display. there has been no relevant research about the effect of environmental illuminance on visual performance of mobile phone and the optimal parameters of phone screen brightness under different environmental illuminance. In the present study, behavioral performance, subjective evaluation and eye fatigue indices were used to examine the impact of the environmental illumination on the visual search task performance on cell phone. There were totally 64 undergraduates participated in a visual search task with 16 subjects took part in the experiment 1and the other took part in the experiment 2.They were all college students with normal or corrected normal vision and all were right-handed. Experiments carried out in the quiet 3x5m2 laboratory. The light in the laboratory could be adjusted to control environment illumination with the maximum illumination value could achieve 500lx. Additionally, a touch screen mobile phone (Samsung S5660 with 3.2 inch TFT-LCD screen, resolution 480x320), PR880 luminance meter, YF2006 illuminometer, BD-II-118 type flash fusion frequency instrument were used in the study. In experiment1, when the phone screen brightness remained unchanged, the visual search performances under different levels of environmental illumination were investigated. Single-factor within-subjects designs were adopted with independent variable including ambient illumination in three levels: 0lx, 100lx, 500lx. After practice, participants were asked to search and count the number of target in different experiment conditions. The search time and accuracy, subjective evaluation and the fatigue of eyes were recorded. In experiment2, there were two independent variables including ambient illuminations: 0lx, 100lx, 500lx and mobile phone screen brightness degrees: 10%, 20%, 40%, 60%, 80% and 100%. Two factors mix designs were adopted in experiment 2 and the procedure seemed like experiment 1. The results showed that when the phone screen brightness remained the same, the visual search performance, subjective evaluation and the degree of eye fatigue were significantly differenced in different ambient illumination and the performance in 100lx were better than 0lx and 500lx. Experiment2 results showed that when the ambient illumination was 0lx, the best screen brightness degree was 20% with the brightness value was 11cd/m2, when the ambient illumination was near 100lx, the recommended screen brightness degree was 40% with the brightness value was 68cd/m2 and when the ambient illumination was near 500lx, the recommended screen brightness degree was 60% with the brightness value was near 257cd/m2.