Isolated cold aisle airflow distribution is a transitional form from non-isolated airflow distribution to closed cold aisle airflow distribution. With the increase of the power of racks, cooling failure may happen in the layout of the isolated cold aisle. This paper presents the study on cooling performance of the racks which are improved through bottom ventilating reform and adjustment. Inlet/outlet air temperature and mass flow rate of the racks are investigated in detail under conditions of various bottom ventilated areas and various porosities of the raised floor. The characteristics of airflow distribution are contrastively analyzed through calculating indexes of the thermal environment of data centers. Results show that adequate ventilation through the bottom of the racks is good for improvement of the state of airflow distribution. There is an optimization range (0.1–0.3 m and 0.05–0.15 m, respectively) of the ventilated area at the bottom of the racks. And high porosity (above 50%) of the ventilated area can reduce the inlet and outlet temperatures of the racks and the racks in different positions have a better temperature uniformity. In conclusion, bottom ventilation of racks is a feasible plan to improve airflow distribution, and schemes of ventilated area and porosity of corresponding raised floor should be designed respectively under consideration of the layout of racks and AC.