Fume hoods, when properly connected to the laboratory ventilation system, can effectively remove harmful gases generated during the experiment to protect the safety of laboratory personnel. Laboratory fume hoods must be equipped with a deflector plate a BYPASS system to control the shape of the airflow inside the hood the distribution of the airflow at the openings. Fume hoods are equipped with BYPASS holes above the sliding door, when the sliding door is lowered the BYPASS holes open to allow outside air to make up for the cabinet to maintain the surface air velocity, maintain the balance between the indoor ventilation system the volume of gas consumed by the fume hoods, impede the increase in the surface air velocity due to the lowering of the sliding door to avoid the impact on the operator.