Abstract: The paper proposed a boundary extraction algorithm based on sub image neighborhood gradient information metrics. By constructing the neighborhood information mapping matrix and using the metric of matrix gradient information, the proposed algorithm extracted the effective border. The methods and procedures were analyzed for boundary detection and contact angle measurements of the image of poor droplet. Experimental results show that this algorithm can effectively extract the object boundaries of the image of poor droplet and achieve the automatic calculation of contact angle, whose results are consistent with the manual calculations.