Performing dual task may impair balance and gait in cerebral palsy (CP). But the effect of dual task on the electromyographic activity of CP patients has not been well studied yet. The aim of this study was to investigate the electromyography (EMG) activity of the lower limbs and erector spinae muscles in CP patients and healthy controls during walking with and without a cognitive dual task. To do so, the EMG activity of biceps femoris (BF), rectus femoris (RF) and erector spinae at L3 level (L3) muscles of 10 CP patients and 10 healthy controls were recorded using an EMG USB2+ system and bipolar electrodes during walking with and without the cognitive task. Data were statistically analyzed using SPSS 22 and ANOVA for repeated measures with a significance level of 0.05. Results showed that during walking with the cognitive dual task, the EMG activity of the left L3, BF and RF muscles and right L3 and BF muscles was significantly higher than that of the normal subjects. The pattern of normalized EMG activity of L3, BF and RF muscles of CP patients and healthy control subjects differed during gait with and without the cognitive dual task, and there was a significant interaction between the group, muscle and task factors. Based on our results, cognitive dual task increases EMG activity of CP patients more than healthy subjects. This increase can be attributed to the effort of CP patients to maintain posture and dynamic balance. Thus, it is recommended to pay attention to the cognitive dual task when evaluating and designing exercise programs for CP people with cerebral palsy.