The present invention relates to an extensible protocol processing system. The system of the invention is mounted between a host and a network, and is utilized to offload partial network protocol packets of the host. The protocol processing system comprises a microprocessor, a host interface device, a medium access controller, a message interface controller, a receiving device and a transmitting device. The system of the invention can provides better efficiency of the host for processing the offloaded partial network protocol packets. Besides, the function of the system of the invention can be extended by increasing the message and modifying the software or the firmware. Therefore, the system of the invention can be utilized to various applications, and has the easily extensible effects.