<p>Ultrasonic fracturing, a novel method, has diverse applications in hard rock boring and oil and gas extraction. Understanding the fracture mechanisms of cracks under ultrasonic loading is essential for advancing the study of ultrasonic fracturing in rocks. This paper introduces an innovative method, 3D-ILC (3D-internal laser-engraved crack), to pre-fabricate coplanar internal cracks inside rock-like materials without damaging the specimen surface, and subsequently apply ultrasonic loading. The results show that, under ultrasonic fields, coplanar cracks are attracted to each other and coalesce at neighboring crack tips due to interaction. The shape of the coalesced crack transitions gradually from peanut-like to ellipsoidal. The “I”-shaped coalescence characteristic between coplanar cracks is caused by the intersection of Wallner lines, which form due to the propagation of stress waves under the ultrasonic field. Numerical simulations based on the Paris fatigue model indicate that the stress intensity factor at the center of the peanut-shaped crack (concave coalescence point) is significantly higher than at other locations and decreases as the crack propagates. Comparing the characteristics of coalesced crack surfaces under uniaxial tensile and thermal loading reveals that the roughness and irregular distribution of Wallner lines on the crack surface under ultrasonic loading are due to multidirectional stress waves induced by ultrasonic propagation within the material.</p>