A quality of service adjusting method based on application categories is configured to adjust a Quality of Service (QoS) of a communication device. A network session processing step is performed to drive a hardware accelerator to process a plurality of network sessions. A packet receiving step is performed to drive a processing unit to receive a first packet group of each of the network sessions. A packet analyzing step is performed to execute a packet inspecting module to analyze a plurality of packet data of the first packet group of each of the network sessions, and to classify an application corresponding to each of the network sessions. A list establishing step is performed to establish a priority list. A bandwidth distributing step is performed to distribute a network bandwidth to a second packet group of each of the network sessions according to the priority list.