Abstract: Assuming the working face in the strip extraction as a series of co-linear cracks with certain spacing interval,the Ⅰ-Ⅱ composite crack model in fracture mechanics was adopted to get the stress distribution at the edge of coal pillar,so the border equation of the edge of coal pillar was created.Accordingly the formula for calculating damaged width of the strip coal pillar was obtained.It was shown that the damaged width calculated by the formula was in accordance with the observed data.