The advanced rotational hydrodynamic cavitation reactor (ARHCR) has satisfactory effectiveness for process intensification, due to intense cavitation caused by strong shear force between rotor and stator. In this paper, the internal cavitation mechanism of a representative ARHCR was studied by analyzing the results from high-speed photography, pressure pulsation, and acceleration for three representative operating conditions of non-cavitation (1100 rpm), inception cavitation (1800 rpm), and critical cavitation (3000 rpm) for 2.16, 2.4, and 3 m3/h. The results show that the pressure pulsation was positively correlated with the cavitation intensity, with a dominant frequency of the pressure pulsation of 1600 Hz.