The use of radial jet drilling (RJD) to exploit unconventional natural gas is an attractive study area. A self‐propelled bit enormously affects the drilling efficiency. Herein, for a straight‐swirling mixed jet (SSMJ) bit, the internal and external velocity fields were analyzed using a simulation to reveal a jet mixed mechanism. The rock‐breaking characteristics of different bit structures were investigated using laboratory experiments. A straight jet and a swirling jet were independent flowing through the impeller and were gradually mixed together in the mixing chamber inside the bit. The jets' boundary gradually dissipated along the radial direction, and the swirling jet distribution along the circumferential direction tended to be uniform. The velocity vector gradually decreased from the center to the periphery, and the energies were slowly concentrated due to the cluster‐narrowing by the conical wall until the velocity at the nozzle outlet reached its peak. Different flow fields were modulated by changing the bit structures. The diameters ratio greatly affected the distribution proportion of straight and swirling; the length‐diameter ratio primarily affected the contribution of the swirling jet to its center, and the width‐diameter ratio chiefly affected the initial formation velocity of the swirling jet, which all directly affected the depth variation along the radial and diameter of a borehole. The hole depth and diameter, erosion volume, and bottom flatness were combined to evaluate the rock‐breaking performance. The bit with a diameters ratio of 0.67, a length‐radius ratio of 0.57, and the width‐radius ratio of 0.13 had the best erosion effect. The simulation and experimental results achieved good mutual verifications. Overall, this study can guide research on the SSMJ rock‐breaking mechanism and bit design for RJD.

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