Abstract In this study, bench tests of a heavy-duty turbocharged natural gas spark ignition (NGSI) engine were conducted with intake air injection at full load and different engine speeds. The flow characteristic of the compressor was revealed. The flow capacity of the compressor is reduced with air injection, and the reduction range increases gradually with the air injection pressure increasing at a constant speed, which would likely lead the compressor to surge. But the decrease extent of the compressor flow rate is improved as the speed increases, which reduces the tendency of surge. Based on those, prediction models for safe air injection pressure which can avoid compressor surge during various operations were proposed and then validated with experimental data. In addition, the influence of air injection on the pumping loss was also analyzed. The turbocharger efficiency is reduced therefore the pumping loss of the engine is increased during the air injection process. At 1200 rpm, the pumping loss efficiency of the engine without air injection is 0.25%, while it is increased to 1.83% with an air injection pressure of 400kPa at the same load.