Embodiments of the present invention provide a method for controlling packet access, a network side device, a terminal device and a communication system. The method includes: generating an access trigger message, where the access trigger message carries a temporary mobile group identity TMGI allocated to a user equipment UE; and sending the access trigger message to packet trigger an access operation of the UE for the TMGI. In the embodiments of the present invention, the group identity TMGI already existing in an MBMS can be used as the group identity of packet triggering UE access, and the access trigger message carrying the TMGI can be correspondingly generated and sent so as to trigger an access operation of the UE belonging to the TMGI, thereby implementing the function of the packet triggering user access.