Abstract: Starting from the point of view to solve the difficulties of drilling, the paper studies the well wall protecting slurry in coal-bearing complex formation. Through the indoor test study of bentonite-polymer slurry system, then adopting the method of orthogonal test, the optimization formula of wall-protecting slurry was obtained. Finally the slurry of high polymer of low solid phase was used in field test, achieved good results in well wall protection during drilling in coal-bearing complex formation.