In order to improve reality of pencil sketching, convolution threshold algorithm is established. Streamline of pencil is generated. Image texture is generated with function of line integral convolution, streamline is integrated by pixel along the vector and negative direction obtained, input noise of all pixel of streamline as kernel are convoluted, output texture pixel value is obtained. Different ranges of gray are processed by two different thresholds of gray，and process is described. The experiment shows this algorithm made edge of pencil sketching clearly and delicately, and has pencil texture, processing time is 2.2 s, it takes less time and is more like hand-painted.