Flow in curved duct is different from that happens in straight pipeline. Affected by the curved duct curvature, the flow field in a curved duct is uneven. The flow in the curved duct suffers serious loss. Considering the flow field characteristic and engineering applications, the characteristic of the flow field in curved duct without guide plate and the curved duct with different guide plates have been studied based on the numerical results. The velocity distribution and unevenness on the outlet of the curved duct have been analyzed too. Then the best conditions for guide plates installed in curved duct are achieved.