The double blade pump is widely used in sewage treatment industry,however,the research on the internal flow characteristics of the double blade pump with particle image velocimetry(PIV) technology is very little at present.To reveal inner flow characteristics in double blade pump impeller under off-design and design conditions,inner flows in a double blade pump impeller,whose specific speed is 111,are measured under the five off-design conditions and design condition by using 3D PIV test technology.In order to ensure the accuracy of the 3D PIV test,the external trigger synchronization system which makes use of fiber optic and equivalent calibration method are applied.The 3D PIV relative velocity synthesis procedure is compiled by using Visual C++ 2005.Then absolute velocity distribution and relative velocity distribution in the double blade pump impeller are obtained.Test results show that vortex exists in each condition,but the location,size and velocity of vortex core are different.Average absolute velocity value of impeller outlet increases at first,then decreases,and then increases again with increase of flow rate.Again average relative velocity values under 0.4,0.8,and 1.2 design condition are higher than that under 1.0 design condition,while under 0.6 and 1.4 design condition it is lower.Under low flow rate conditions,radial vectors of absolute velocities at impeller outlet and blade inlet near the pump shaft decrease with increase of flow rate,while that of relative velocities at the suction side near the pump shaft decreases.Radial vectors of absolute velocities and relative velocities change slightly under the two large flow rate conditions.The research results can be applied to instruct the hydraulic optimization design of double blade pumps. The double blade pump is widely used in sewage treatment industry, however, the research on the internal flow characteristics of the double blade pump with particle image velocimetry (PIV) technology is very little at present. To reveal inner flow characteristics in double blade pump impeller under off-design and design conditions, inner flows in a double blade pump impeller, whose specific speed is 111, are measured under the five off-design conditions and design condition by using 3D PIV test technology. order to ensure the accuracy of the 3D PIV test, the external trigger synchronization system which makes use of fiber optic and equivalent calibration method are applied. The 3D PIV relative velocity synthesis procedure is compiled by using Visual C ++ 2005. Chen absolute velocity distribution and relative velocity distribution in the double blade pump impeller are obtained.Test results show that vortex exists in each condition, but the location, size and velocity of vortex core are different .A verage absolute velocity value of impeller outlet increases at first, then decreases, and then increases again with increase of flow rate. Again average relative velocity values ​​under 0.4,0.8, and 1.2 design condition is higher than that under 1.0 design condition, while under 0.6 and 1.4 design condition it is lower.Under low flow rate conditions, radial vectors of absolute velocities at impeller outlet and blade inlet near the pump shaft decrease with increase of flow rate, while that of relative velocities at while .Radial vectors of absolute velocities and relative velocities change slightly under the two large flow rate conditions. The research results can be applied to instruct the hydraulic optimization design of double blade pumps.