Objective:In order to explore the maximum and the optimal velocity of front window air in biological safety cabinets and provide a scientific evidence for relevant test. Methods: As classⅡ biological safety cabinet an testing sample, computational fluid dynamics method is adopted to establish numerical model, then made the simulation of front window airflow. Results: The velocity relationship between front window airflow and the internal air has been confirmed. Conclusion:The maximum velocity of the front window flow is 1.4 m/s and the optimal velocity is 1.0 m/s. The tested data and conclusion can give some suggestions and reference for biological safety cabinets detection.