Abstract: Based on the technical problems encountered during the development of BH-VDT3000 vertical drilling system (VDS), the influencing factors and rules of its straightening capability were analyzed by the prediction method of equilibrium trend build-up rate. The reasons for the field test failure of the first-generation BH−VDT3000 prototype were comprehensively analyzed, thereafter the optimization scheme of the structure, bottom-hole assembly (BHA), and drilling parameters of the second-generation prototype were proposed. Field tests were also conducted. The theoretical research and field test results show that the bit lateral force of the push-the-bit VDS pointing to the low side of the borehole has positive impact on straightening, and the bit tilt angle pointing to the high side has passive impact on straightening. Their influences on the drilling trend should be balanced when selecting the BHA and drilling parameters (including the push-the-bit force), otherwise, the straightening capability will be lost. Because BH-VDT3000 VDS has a small diameter and low stiffness, it is very likely to lose the straightening capability if the same BHA of a large-size VDS is used. Shortening the overall length of the steering sub and the distance between the steering pads and the stabilizer can improve the straightening capability of the medium-size VDS.