The invention relates to a dynamic measurement device of material laser reflectivity under a subsonic tangential airflow. The device comprises a collection and measuring system and a test section. The test section comprises a test material placement zone, a blowing system, and a semi-ellipsoid reflectometer. The test material placement zone and a measuring port of the collection and measuring system are respectively positioned at a first focal point and a second focal point of the semi-ellipsoid reflectometer. The test section also comprises an air channel connected with the lowing system. The air channel comprises an air channel air outlet port and an air channel air inlet port. An air outlet direction of the air channel is parallel to the outer surface of a test material placed at the test material placement zone. The invention also provides a dynamic measurement method of the material laser reflectivity under the subsonic tangential airflow, based on the device. With the device and method provided by the invention, a material reflectivity dynamic change trend curve during the interaction of laser and metal material under the condition of subsonic tangential airflow can be obtained.