The engine related parameters， such as intake manifold pressure， air/fuel ratio（AFR）， fuel mass flow-rate， as well as dynamic in-cylinder pressure traces， were measured on a turbocharged LNG engine with external air injection according to the transient procedure. During the transient and ramped modal testing， the engine experienced not only abrupt change of boost pressure and AFR， but also rapid engine load change. The changes in the in-cylinder combustion process during the transient test cycle， which were rarely revealed before， were illustrated with the rate of heat release（ROHR） characteristics. The influence of operation parameters on the combustion process under transient conditions was examined and discussed. Moreover， the existence of abnormal combustion such as late combustion， misfiring and severe variation of IMEP， etc. was pointed out. The possible cause of those abnormal combustion phenomena was also analyzed. Results show that under steady state and transient conditions， the LNG engine combustion process is dominated by the relative AFR.