The Longmaxi Formation shale in the Sichuan Basin has a large burial depth, narrow horizontal windows, and stringent requirements for controlling well deviations. When PDC bits are used for drilling, the cutting structure experiences significant challenges in terms of stability and rock-breaking efficiency. The secondary design cycle of PDC bits is lengthy, and the cost of engineering trials is high. Thus, bit development relies on strong empirical knowledge. Additionally, the spiral degree of the blades has not been uniformly defined, and there was a lack of quantitative studies about its effect on the rock-breaking performance of PDC bits. In this study, thus Archimedean spiral was used as the quantitative control equation for the spiral degree of the blades. The cutting depth control distance ΔL2 (or designed depth of cutting, designed DOC) and the level of different track (LODT) were used to adjust the radial cutting section of the interaction process between the bit and shale. An orthogonal coupling study was conducted on the rock-breaking efficiency and stability of 8 1/2ʺ PDC bits with six spiral span angles θspan, six cutting depth control distances ΔL2, and six LODTs as design parameters. The results indicated that using the triaxial compressive strength under the liquid column pressure as the mechanical specific energy (MSE) benchmark, the shear failure mode reduced the mechanical specific energy below this benchmark when cutting depth control distance ΔL2 of the front and rear elements exceeded 1.0 mm. However, increasing the cutting depth control distance ΔL2 led to an increase in torque and circumferential vibration amplitude. In addition, compared to straight blade designs, spiral blade designs significantly reduced the axial/circumferential vibrations, with reductions in circumferential vibration amplitude ranging from 37.87% to 48.33%. The spiral blade designs had a suppressive effect on axial vibrations only when the spiral span angle θspan was between 12 and 36° or 60°, reducing the axial vibration amplitude by approximately 20.09–25.69%. Different ΔL2 must be considered in conjunction with cutter material properties when selecting the spiral span angle based on the simulation results. When the ΔL2 exceeded 1.0 mm, LODT of 3/6 could maximize the reduction in the mechanical specific energy. Using MSE as the objective function, this study provided several PDC bit enhancement design schemes suitable for the Longmaxi Formation. The research results contributed to a quantitative understanding of the coupling effects between spiral span angle θspan and cutting sections on the rock-breaking efficiency and stability of PDC bits.