Abstract: Automatic vertical drilling system (VDS) is a special system and high-end drilling equipment for fast vertical drilling. In view of the technical problems encountered in the development of BH-VDT 3000 of Bohai Drilling Company, the influence factors and rules of the straightening capability are predicted and analyzed by the prediction method of equilibrium trend build-up rate. The reasons to the field test failure of the first prototype are comprehensively analyzed, and then the optimization scheme of the structure, bottom-hole assembly (BHA) and drilling parameters of the second prototype and its field test results are presented. The research results have shown that the bit lateral force of the push-back vertical drilling system points to the low side of the borehole (it is conducive to straightening) and the bit tilt angle points to the high side (it is unfavorable to straightening). Their influences on the drilling trend should be balanced when selecting the BHA and drilling parameters (including the push force), otherwise the straightening capability will be lost. Because the VDT 3000 has a small diameter and small stiffness, if the same BHA of large size VDT tool is used, it is very likely to lose the straightening capability. In this case, the greater the push force, the faster the inclination increase. Shortening the length of the VDT tool and the distance between the steerable rib and the upper stabilizer can improve the straightening capability of the medium-size vertical drilling system.