Abstract： This paper studied the CO, HC and NOx emission characteristics of high-pressure direct-injection natural gas engine at different diesel rail pressures and gas injection timing by a series of experiments. The experimental results show that, at idle operation, diesel rail pressure has little influence on CO emissions when the gas is injected near top dead center (TDC). CO emissions decrease with the diesel rail pressure when the gas is injected at a certain angle after TDC and increase with the diesel rail pressure when the gas is injected at a certain angle before TDC. The advancing of gas injection results in lower CO emissions at three different diesel rail pressures. HC emissions increase with the rising of diesel rail pressure and reduce with the advancing of gas injection timing, both increasing significantly when the diesel rail pressure rises from 18 MPa to 24 MPa. NOx emissions  increase  with the advancing of gas injection, and increase markedly when the diesel rail pressure rises from 12 MPa to 18 MPa.