A packet transmission method, apparatus, and system in a multicast domain name system (mDNS), which are used to resolve a network bandwidth waste problem due to much service information needs to be transmitted in an mDNS-based network. The method includes, converting, by a relay, a known-answer service query packet that is sent by a user terminal and used to query service information in an mDNS, into a unicast known-answer service query packet and sending the unicast known-answer service query packet to a gateway, receiving, by the relay, a unicast known-answer service answer packet that is sent by the gateway and includes service information unknown to the user terminal, and converting, by the relay, the unicast known-answer service answer packet into a multicast known-answer service answer packet and sending the multicast known-answer service answer packet to the user terminal.