The evaluation index, such as spray combustion, gas mixing and nozzle non-uniformity of air heater based on the combustor of liquid rocket engine, was studied with numerical simulation. The difference between the injectors with ethanol/liquid oxygen/air and the injectors with ethanol/gaseous oxygen/air in the combustion flow field was discussed and analyzed. Two cases were designed by changing the characteristic length of combustor, and the performance of air heaters were investigated by numerical simulation. The results show that, the flame structure is affected obviously by the phase of oxygen. The length of flame with liquid oxygen is larger than that of gaseous oxygen. More hot gas fills in the recirculation zone in the forepart of combustor with gaseous oxygen case, which increases the gas temperature near the faceplate, and worsens the thermal environment on the faceplate. The high quality flow field on the nozzle exit is obtained with the designed air heater. The characteristic length of air heater with gaseous oxygen is at least a quarter less than that of air heater with liquid oxygen to keep the good uniformity of nozzle exit.