The design method of small-flow high-head centrifugal-vortex pump was presented. This pump, configured with inducer, complex-centrifugal impeller and open-vortex impeller, was put forward to deliver gas-liquid two-phase mixture. An HTB-5/60 type sample pump was developed and tested on a closed-loop test rig. Experimental studies on performance and cavitation tests for gas-liquid two-phase mixture were carried out compared with pure-water experimental results. Also the effect of gas phase on pump was analyzed and discussed. The experimental results show that performance and cavitation characteristics of the sample pump deteriorates progressively with increasing volume fraction of gas. When the total capacity Q(m) is between 4.5 m(3) center dot h(-1) and 6 m(3) center dot h(-1) and the gas flow rate q(g) below 0.66 m(3) center dot h(-1) or q(g)/Q(m) is lower than 15%, the characteristic curves are approximately parallel to those in pure water test, but the performance deteriorates sharply until an abrupt flow-cutting at a critical volume fraction of gas. This pump is found suitable for transporting gas-liquid two-phase mixture when working around rated capacity of 5 m(3) center dot h(-1) with q(g)/Q(m) below 15%.