During drilling operation of Dongying formation in Bohai oilfield, mudstone is hard and brittle, hole wall collapses easily, anticline structure and rock mechanics are complex, mechanical drilling rate is low, borehole is long, hard and brittle mudstone flakes off in downhole accident. The high density of cuttings per unit annulus volume leads to pressure suppression, large torque fluctuation, and drilling jam during drilling. The drill tool breaks and falls during the processing of drilling jam, and the drilling tool of 125.94 m bottom is successfully salvaged after casing milling, and the through well cannot be reached to the end. It is decided to backfill sidetracking. After the success of sidetracking, the performance of PEM drilling fluid is optimized and new tools and technologies are used. Finally, no accident of tripping, casing, logging and cementing operation smoothly, ensure the well structure quality. The experience and lessons of drilling jam accident in this well have guiding significance and popularizing value for further deep well development in this block.