Abstract: The backward-facing step flow is a simple structure but has complex flow patterns. In this paper， the backward-facing step flow is simulated by a 2D k-ε turbulence model. And based on the test results measured by PIV system， we analyzed the flow field data of the backward facing step for Reynold number from 500 to 50，000. The research results show that the flow field structure has a steady recirculation zone， and that the length of the recirculation zone increases rapidly with the increace of Re in the front， then rapidly declines， and finally slowly rebounds and becomes stable. The cross section velocity is affected greatly by the recirculation zone， and the peak value decreases slowly with the increase of Re. The pressure and turbulence kinetic energy are also affected by the recirculation zone， but the turbulence kinetic energy is less affected in low Reynolds.