To investigate the outflow characteristics of the trapezoidal sluice gate, a novel type of flat gate, model experiments and dimensional analysis were used to conduct experimental studies and theoretical analyses on the flow characteristics of submerged flow through the trapezoidal sluice gate. The experimental results indicate that when the submerged outflow occurs downstream of the trapezoidal sluice gate, the greater the degree of submergence is, the closer the contracted section is to the gate, the smaller the transverse water surface difference at the contracted section is, and the shorter the distance required for the transition from rapidly varied flow to gradually varied flow downstream of the gate is. Based on flow pattern analysis, combined with dimensional analysis and numerical fitting, the flow pattern discrimination criteria for the trapezoidal sluice gate were obtained, and an equation for calculating the submerged discharge was derived. The evaluation and verification of this equation show that the average relative error is within 5%, indicating a high level of calculation accuracy.