Recently,　the　continuous　tube-in-tube　reactor　based　on　the　Teflon　AF　membrane　is　emerging　as　a　powerful　toolkit　for　accelerating　gas-liquid　mass　transfer　and　reaction　rate.　Because　of　its　large　gas-liquid　interfacial　area　and　short　mass　transfer　distance,　the　reactor　can　allow　a　fast　gas-liquid　mass　transfer　without　direct　contact　between　gas　and　liquid　phases,　offering　an　efficient　and　safe　platform　for　implementing　gas-liquid　reaction　and　rapid　determination　of　gas-liquid　parameters.　In　this　review,　a　detailed　description　and　construction　method　of　this　reactor　are　provided.　Then,　the　recent　advancements　of　the　tube-in-tube　reactor　in　fundamental　studies　and　practical　applications　in　gas-involved　chemical　reactions　and　biosynthetic　processes　are　discussed.　Finally,　a　perspective　on　future　potential　applications　of　such　flow　reactors　is　provided.