Abstract: Gas turbine combined cycle systems(GTCC) have wide applications to utilize byproduct gases with the high efficiency in China's steel plants. An analytical model was proposed for simulating the operation of a low calorific value gas GTCC, aiming to analyze the influence of ambient temperature, lower heating value(LHV) of gas, and inlet air temperature on the combined cycle performance under off-design conditions. Three operating schemes for improving the performance of GTCC were evaluated as the ambient temperature rises. The research results show that the maximum output of the combined cycle can be achieved through the operation scheme combining the regulation of inlet air temperature and LHV. For the 300MW M701S(F) unit, the combined cycle output is increased by 28.96MW through inlet air cooling at ambient temperature of 35℃. When the regulation of gas LHV is considered simultaneously, the combined cycle output can be ultimately increased by 30.28MW.