Abstract: To evaluate the image quality without reference under blocking effect，the author proposed a bidirectional filtration （BIF） method，which aims at providing a new idea for boundary selection，and resolved the image evaluation under blocking effect more effectively by distinguishing the true and false boundary of the image. The method filtered the image boundary layer by layer from two different fields to locate the true boundary effectively. Finally，the evaluation values were obtained through the evaluation model. Compared with SSIM model，the data obtained from the experimental results were improved to a certain extent. It is concluded that this method is more accurate in evaluating the no-reference image under blocking effect.

Key words: bidirectional filtration, image quality assessment, image boundary