One of the subjects being discussed by the professional community currently is the role possibly played by the mirror neuron system (MNS) in the violation of social behavior of children with autism spectrum disorders (ASD). The MNS is known to shape the perception of emotions of others and understanding and imitation of their actions. Mu rhythm desynchronization in EEG is considered to be the indicator of the MNS activation. The purpose of this study was to identify the features of reactivity of the EEG mu rhythm within an individually determined frequency range in preschoolers with ASD in situations requiring instrumental, emotional and altruistic helping behavior (HB). The study involved children 4–7 years old with ASD (n = 26) and their normally developing peers without the condition (n = 37). Although in most cases, HB was more pronounced in the group of normally developing children, the differences between the groups are significant only for altruistic HP (p < 0.01), and for the situation requiring complex altruistic and emotional HP it approaches significance (p = 0.09). Evaluation of the mu rhythm reactivity indices showed that the tasks invoking complex altruistic and emotional HB bring this indicator down significantly in children with ASD compared to the group of normally developing participants, as shown by the central leads of the left and right hemispheres and the parietal lead of the right hemisphere (C3: p = 0.02 ; C4: p = 0.03; P4: p = 0.03). It is assumed that the detected features stem from the impaired functioning of the MNS and the downstream regulation to the MNS from prefrontal cortex and other areas of the neocortex. The data obtained can be used in development of EEG biofeedback training protocols for children with ASD.