A novel heating technology is presented to analyze the influence of inlet air heating on gas turbine efficiency under partial load. This technology uses the waste heat of a low-temperature heat sources, which includes but is not limited to the exhaust gas of a combined-cycle heat-recovery steam generator or a single-cycle gas turbine. A calculation model of the equilibrium running point is used for the given load and the characteristic curves of the compressor and the turbine to study the mechanism of the inlet air heating technology. Then, the equilibrium running line is calculated and drawn in the characteristic curves of the compressor and the turbine. The factors for gas turbine efficiency are discussed through the calculated equilibrium running line. The results show that an increase in inlet air temperature has considerable potential for improving gas turbine efficiency due to the increase in compressor and turbine efficiency. This finding is different from traditional viewpoints. Meanwhile, each partial load has an optimum heating temperature which becomes higher when the load is lower.