Abstract: Deep strata play an important role for the reserve growth of Bohai Oilfield.The mud shale in deep strata which is mainly composed of non-expansion clay minerals (e.g.illite and illite/smectite mixed layer) is poor in hydrate expansion and dispersion capacity, and the short-range repulsion of hydration film can lead to the deformation of hydration interface.Thus, borehole collapse and instability tends to happen easily.In this paper, according to the micro- and nano-scale instability mechanisms, the "multi-element collaboration"countermeasures for drilling fluid stability and borehole collapse prevention were put forward to deal with the peculiar borehole instability forms in the deep strata of Bohai Oilfield.The countermeasures include physicochemical plugging, inhibition enhancement, activity equilibrium and rational density support.Then, they were applied practically.And thanks to their application, the borehole instability in the middle and deep mud shale of Dongying Formation and Shahejie Formation in Bozhong Block and Penglai 7-6 Block is solved.