The invention provides a message sending method, a device and a system. The message sending method comprises the following steps of receiving a designated Internet designated control message protocolICMP message sent by a network side edge PE node by a client side edge CE node, and determining whether the designated ICMP message carries a mark indicating that a underlying network does not supportfragmentation; under the condition that a determination result is positive, using a maximum transmission unit MTU in the designated ICMP message to generate a path maximum transmission unit PMTU which does not support fragmentation; and carrying out fragmentation sending on an IPV6 message whose length is greater than that of the PMTU which does not support fragmentation. In a correlation technology, When a MPLS domain MTU is less than 1280 in a 6PE network, a flow is blocked. In the invention, the above problem is solved, simultaneously, when MTU configuration is too small, the effect of flow conduction can still be achieved.