According to the characteristic of high concentration of red pixels in pentagram area, a new method of circular seal imprint extraction based on smoothing convolution location is proposed. Firstly, the colorful bill image is converted to grayscale image. Secondly, a pixel in pentagram area is located by smoothing convolution, and then the grayscale image is converted to binary image using the average of grayscale value of the neighborhood of the pixel as the threshold. Finally, the seal imprint is extracted after calculating the coordinate of the center and the radius of the circular seal imprint by searching its outmost frame. Meanwhile, in order to improve the extraction speed, the algorithm of smoothing convolution, with the highest time complexity in the whole extraction process, is optimized by reducing the image resolution appropriately during convolution. Experiments show that the method can effectively extract the circular seal imprint from complex background. And the computation speed increases by nearly 12 times after algorithm optimization.