A ventilation jet diffuser is characterized by parameters such as diffuser effective area, diffuser dimension, diffuser position, air supply direction, flow rate, and air temperature. This paper studies the influence of the parameters of a jet diffuser on the airflow pattern, indoor air quality, and draft risk in an office with a jet diffuser on the rear wall near the ceiling. The presentation of furniture and occupants in the office is included in the numerical simulation. The structure of a jet diffuser used in practice is complicated. Therefore, a simplified method is introduced to simulate the diffuser. The method is compared with the experimental data. The agreement between the simulations and the measurements is reasonably good. The distributions of the air velocity, temperature, contaminant concentration, and percentage dissatisfied people due to draft risk with different parameters of the diffuser are numerically predicted by the k-s model of turbulence. The effect of turbulence intensity is taken into account in the computation of draft risk. It has been found that the angle between the jet and the ceiling should be in the range from 20 to 60°C. The effective flow area has a strong impact on the indoor airflow pattern since it significantly affects the throw projection. The diffuser width has a larger influence on indoor air diffusion than the diffuser height. The distance between the inlet and ceiling has a remarkable influence on the total air movement near the ceiling, but has a minor impact on the air diffusion in the occupied zone. Air velocity distribution is sensitive to ventilation rate and supply air temperature. To achieve the same length of throw projection, the Reynolds number should be the same if the corresponding Archimedes number is close to each of them.