Abstract:  An off-design performance test investigation on humid air turbine (HAT) cycle was conducted on a small-size two-shaft gas turbine test rig. The goals of this paper are to study the gas turbine performance improvement in HAT cycle at low parameters. Two different conditions are taken into account for the test investigation: in Case I, the fuel flow was kept constant while in Case II the turbine inlet temperature was kept constant. In Case I, when the air humidity ratio was increased, the power output was increased, and at the same time, the turbine inlet temperature (TIT) and the NOx emission were reduced. In Case II, when the air humidity ratio was increased, the power output could be enormously increased.Based on the gas turbine parts characteristics and test conditions, the off-design performance of HAT cycle was calculated and the results show that the HAT cycle performs better than twoshaft cycle in terms of the specific work and the efficiency, and the effect is more obvious along with the increase of air humidity ratio. Increasing humidity ratio can also enlarge surge margin on compressor curves at the same inlet air flow, turbine inlet temperature and power output. Key words：

Key words: humid air turbine (HAT) cycle, experimental investigation, off-design performance, humidity ratio, surge margin