Abstract: The development of shale gas in the Qiongzhusi Formation requires directional build-up drilling in the Douposi, Longwangmiao, and Canglangpu Formations (collectively referred to as bad grounds). This formation interval exhibits high hardness and high abrasiveness, resulting in an extremely low rate of penetrate of only 0.88 m/h during build-up drilling. To address this technical challenge, on the basis of outcrop drillability experiments on difficult-to-drill formations, a calculation model for bit side-cutting capability and build-up rate was established, taking into consideration of rock mechanical characteristics and bit force state. The side-cutting and directional steering capacity of bits under varying weight on bit, rotary speed and flow rate were systematically analyzed. With the goal of maximizing side-cutting capability and build-up efficiency, parameters sensitivity analysis and extremum seeking optimization method were engaged to optimize the optimal drilling parameters intervals, which were subsequently validated in field applications. Experimental studies show that the formation lithology is dominant factor influencing side-cutting capability. The average side cutting capability of the bit in the Longwangmiao Formation is 1.16, which is superior to 1.11 in the Douposi Formation. The side cutting capability in rotary steering drilling in the Canglangpu Formation is 1.29, which is 18.3% higher than 1.09 in compound drilling. Weight on bit, rotary speed and flow rate synergistically affect the bit side-cutting capability. The optimum parameters intervals in this region of interest are weight on bit of 110-140 kN, rotary speed of 55-70r/min, and flow rate of 33-35 L/s, achieving high-efficiency side cutting effects and build-up efficiency. Field practices demonstrate that maintaining hole deviation within the range from 20° to 35°in bad ground interval achieves effective build-up rates while extending bit service life. After parameter optimization in bad ground, the maximum rate of penetration increased by over 40%, and the drilling cycle was shortened by 36% compared to the average level. This model enables quantitative evaluation of side-cutting capability and build-up efficiency of bit in various bad grounds, which provides a technical basis and engineering reference for the directional drilling parameters design in highly abrasive and hard formations.