The combined straight jet with swirling jet, generated by combined round straight jet with swirling jet nozzle with a disk, is a new kind of high-efficient jet. The disk has a central orifice and three spiral slots. When high-velocity fluid flows through the disk, the axial motion of fluid is created by the central orifice, while the swirling motion of fluid is created by the slots. When the fluid with both axial and swirling motion flows out of the nozzle, the combined round straight jet with swirling jet is created. The combined round straight jet with swirling jet inherits both the large impact area of a swirling jet and the deep penetration depth of a round straight jet. In order to break rock more effectively, the influences of nozzle structural parameters, standoff distance, and jet pressure on the rock erosion effect were researched by a rock breaking experiment. The experimental results showed that the combined round straight jet with swirling jet nozzle with diameter of 2 mm, chamber length of 8 mm, and extension angle of 60 deg can generate the largest rock breaking volume. The optimal standoff distance is 12 mm. The rock breaking area and penetration depth increased sharply with the increase of jet pressure.