The hydraulic gradients of the small-hole sieve trays with and without slots were experimentally determined. On the basis of the experimental results, a semitheoretical two-layer model was proposed to describe the gradient of clear liquid height on these trays. The model takes into account both the resistance and the driven forces of gases coming from the sieve holes and the slots, respectively. Two model parameters were identified by comparing the calculated results with the experimental data. By applying the model, the open area ratio of the slots on the sieve trays was optimized to give a minimum clear liquid height gradient for various sets of values of design and operating parameters, and the optimum slot open area ratio was correlated with the parameters. The approach developed in this paper can help to manipulate the hydraulic gradient by changing the slot open area ratio.