High-speed diffuse flow is a commonly complex process in tunnels with full-section aerators and lateral expansion walls. The transformation of pressure is prone to produce cavitation on the lateral expansion walls downstream of an aerator. The expansion threatens the stability of the structure and can cause damage. In this paper, the diffuse flows downstream of a radial sluice and full-section aerator were studied by combining a realizable k-epsilon turbulent model with a mixture multiphase model. Hydraulic characteristics such as pressure distribution and cavitation were investigated. The relationship among pressure, cavitation index, lateral divergence angle , and Froude number F were proposed. Empirical formulas were presented to calculate and evaluate the pressure and cavitation index on side walls. The calculated results agree well with the physical model. With decrease in divergence angle, pressure increases gradually so that the flow cavitation index increases as well. Profiles of gate openings demonstrated gradual improvement in pressure and cavitation index with an increase in the magnitude of the gate opening. The studies showed that the degree of gate opening has a great effect on the pressure and flow cavitation index of side walls. The lateral expansion and partial gate-opening increased the risk of cavitation erosion on side walls downstream of the radial sluice. Research results provided a reference for optimizing the design of similar engineering.