Abstract: In order to investigate the distribution law of the air velocity in the sudden expansion roadway and accurately ascertain the boundary of the recirculation zone of the flow field，the particle image velocimetry(PIV)was applied to test the flow field in a rectangular and straight sudden expansion roadway，and the numerical simulation was carried out on the flow field of the sudden expansion roadway under the conditions of different sudden expansion ratio and different inlet air velocity．The research results showed that the streamlines changed from smooth line to mutations while the air flowing through the sudden expansion roadway，the circumfluence whirlpool areas were formed in the upper and lower corner behind the sudden expansion roadway，the existence of the negative wind velocity in the recirculation zone made the velocity distribution in the recirculation zone present a“Ω”type，then the velocity distribution gradually tended to be stable in a“∩”type with the increase of the distance from the sudden expansion transition surface，the position of the sectional air velocity transited from the“Ω”type distribution to the“∩”type distribution was the boundary of the recirculation zone of the sudden expansion roadway; the boundary position of the recirculation zone varied greatly with the increase of the air velocity when the air velocity was in the range of 0．1 m/s to 2．5 m/s，and the boundary position of the recirculation zone had little change with the increase of the air velocity when the air velocity was in the range of 2．5 m/s to 25．0 m/s; when the air velocity was constant，the boundary position of the recirculation zone was farther away from the sudden expansion transition surface with the the incease of the sudden expansion ratio，that was，the recirculation zone became longer．