The present study investigated whether and how background music and noise influences the alertness of a Chinese sample of preschool children on the task of reading picture books. Participants were 36 children aged between 5 and 7 years (25 boys, 11 girls) recruited from a preschool in Nanjing. We observed their off-task behaviors and recorded EEG data while participants paid attention to reading picture books under different conditions: no-noise condition with/without background music and noise condition with/without background music. Results indicated that there was a significant interaction between background music and noise on the general alerting level of the children aged 5–7 years. Concretely, preschool children’s general alerting level without background music is higher than that with background music in the no-noise condition, but the difference disappeared in the noise condition. These findings suggest that background music and noise may have negative effects on children’s alertness and shed lights on stakeholders that a quiet or silent classroom may be the best condition for preschool children to study in.