To investigate how the valence of unattended background picture modulates perceptual processing of a target letter, participants were required to detect a target letter among six letters superimposed on an IAPS picture. Perceptual load was manipulated by varying the difficulty of letter detection, and behavioral results showed more accurate and faster detection responses at the low-load condition than at the high-load condition. The analysis of ERP data of control condition at which six letters were presented without an IAPS picture showed perceptual load effects on mean amplitude of N1 and P1 ERP components. At experimental condition including an IAPS picture, interaction between valence and perceptual load was observed on mean amplitude of N1 at posterior visual processing area, and the amplitude difference between low-load condition and high-load condition was larger on a negative background picture than on a neutral background picture. The results suggest that more attentional capacity might be consumed by a negative picture than by a neutral picture, and suggest that unattended affective picture is processed automatically and influence the early perceptual processing of target stimulus.