In order to solve the bus trail problem that bus route trajectory offset road network and GIS road network information is missing, especially on the rural roads, this paper proposes an algorithm of bus route trajectory based on GIS road networks. Firstly, it makes an in-depth analysis of bus GPS data, clustering line up and down track points respectively. Secondly, it cleans the track points and sort. Thirdly, it combines with GIS road network information for map matching. Finally, according to the improved Dijkstra algorithm, it solves the bus trail problem that GIS road network information is missing. The algorithm is applied in City A with 35 bus lines. The successful match rate is 85%. Unsuccessful matches are due to missing samples or wrong road network information. It can be seen that the algorithm has good accuracy and practicability.