DTH drilling with casing is an effective means to deal with drilling in complex strata. However, the case is usually broken in practical application. Based on the finite element analysis, it is the relation between size and position of case tool escape that causes the case fatigue damage. To solve this problem, pipe shoe was lengthened in order to weaken the impact on case. Compared to the original design, stress and strain of the improved case were dramatically declined with force status greatly improved. The results of field test indicate that the improving scheme is practical and effective.