Drum brake is a braking part which is widely used on heavy truck. The distribution and dimension of contacting pressure and contacting area between friction linings and brake drum have important influence on braking performance. Entire contact theory in this article was found based on the conditions that the outside radius of linings was equal to the inside radius of drum. The relation between the corresponding points on contact surface and the angle which lining rotated around support pin axis was calculated based on entire contact theory. The radial deformation, pressure distribution, contact area of linings and the brake performance were analyzed in the condition of entire contact theory.