In the process of mobile robot path planning, it is difficult for the Rapidly-exploration Random Tree (RRT) algorithm to sample narrow channels. In order to deal with this problem, an improved bridge detection algorithm was proposed, which is dedicated to narrow channel sampling. Firstly, the environment map was pre-processed and the obstacle edge coordinate set was extracted as the sampling space for the bridge detection algorithm, thus avoiding a large number of invalid sampling points and making the sampling points distribution of the narrow channel more rational. Secondly, the process for bridge endpoint construction was improved, and the operation efficiency of the bridge detection algorithm was increased. Finally, a slight variant Connect algorithm was used to expand the narrow channel sample points rapidly. For the narrow channel environment map in the experiment, the improved algorithm has the success rate increased from 68% to 92% compared with the original RRT-Connect algorithm. Experimental results show that the proposed algorithm can sample the narrow channel well and improve the efficiency of path planning.