To solve the problem that traditional Canny algorithm is not accurate to detect the edge of a drop, which leads to a large contact angle measurement error, this paper proposes an improved Canny-Devernay subpixel edge detection algorithm. The algorithm improves traditional Canny algorithm by replaceing gaussian filter with fast-guided filter, calculateing gradient and direction through the operator based on PLIP model, and adopting a local maximum inter-class variance method to achieve adaptive threshold selection. Finally, the improved Canny algorithm is combined with Devernay correction algorithm to realize sub-pixel edge detection. Experiments show that compared with the traditional Canny algorithm, the improved algorithm has more advantages in retaining droplet edge information. When the contact angle measurement method based on this algorithm is detected with a contact angle standard chip, the absolute error and mean of variance can reach as low as 0.011° and 0.009°.