There are some problems in the use of the A* algorithm for path-search of the mobile robot based on a grid map, such as wide search range, calculation slow, search path twists and turns etc. For the characteristics of the four-way mobile robot which moves on the grid map, from the search range, heuristic function constructed, robot acceleration-deceleration and steering costs and other factors on the A* algorithm research and improvement, an extended node algorithm is propose. based on heuristic information, and the number of extensions nodes that deviate from the optimum path is reduced. The results show that the improved A* algorithm can reduce 67.1% of the search area, 49.2% of the computing time in average and reduce 24.9% of the average path cost, as well as decrease 51.1% steering times, and the speed and smoothness of the path-search are improved.