A packet information analysis method and a network traffic monitoring device are disclosed. The method includes the following. Network flow data including a plurality of network packets is obtained. An index parameter is generated according to packet information (e.g., header information) of the first network packet among the network flow. A target mapping model is determined from a plurality of candidate mapping models according to the index parameter. The index parameter is between a first sampling point and a second sampling point of the target mapping model. An interpolation mapping value is obtained according to the index parameter, the first sampling point, the second sampling point, and the target mapping model. An evaluation value is obtained according to the interpolation mapping value. The evaluation value reflects a distribution status of a monitoring item in the network traffic flows.