In order to study the flow characteristics inside the double-channel impeller, a testing pump was designed and produced according to the special requirements for PIV measurement with two different impellers, i.e., the double-channel impeller and the two-blade impeller. The absolute flow fields inside the two impellers were measured respectively by PIV, and transferred to relative flow fields by special developed code. The results indicated that in the two-blade impeller, the fluid mainly flowed along the suction side, the velocity is very small on the pressure side, and a typical jet-wake flow has been observed. In the double-channel impeller, the flow has better flow pattern without obvious back-flow area. Under different operating conditions, the flow inside two impellers changed , with the increase of flow rate, the relative velocity became larger, while the absolute velocity became smaller. For both impellers, the best flow pattern has been observed under the design condition, and serious dispassion occurred under the smallest flow rate working condition.