In this paper, a comprehensive numerical study of the combustion and emission characteristics of a direct injection natural gas engine operating at the high load condition was presented. First, two combustion modes, natural gas mixing-limited combustion (NMLC) and natural gas slightly premixed combustion (NSPC), were defined separately. Subsequently, the potential of combustion chamber type, spray angle of natural gas (SAN), diesel injection pressure (DIP), and natural gas injection pressure (NIP) to achieve high thermal efficiency and low soot emissions are evaluated in both NMLC and NSPC modes. The results showed that in both NMLC and NSPC modes, the straight-sided combustion chamber (SSCC) has more potential to improve the indicated thermal efficiency (ITE) and reduce soot emissions. In NMLC mode, an appropriate increase in SAN can promote the development of vortex at the bowl bottom and improve the stratification state of concentration gradient, thus enhancing ITE, inhibiting the production of pyrene (A4) and acetylene (C2H2), and reducing soot emissions. However, an excessive increase in SAN will lead to fuel accumulation, thus deteriorating soot emissions. In NSPC mode, when SAN increases to 15°, ITE reaches the maximum. Meanwhile, the production of soot precursors (A4 and C2H2) reduces, thus soot emissions reach an extremely low level. However, when SAN exceeds 15°, soot emissions increase rapidly along with the rising SAN. In both combustion modes, when NIP is low, increasing DIP can significantly reduce soot emissions. In NMLC mode, the increase of NIP is more effective in enhancing ITE and inhibiting soot formation. In NSPC mode, when NIP reaches 30 MPa, the maximum pressure rise rate (MPRR) exceeds the limit (1.5 MPa/°CA). When NIP exceeds 30 MPa, ITE begins to decrease. Therefore, NIP should be controlled within 30 MPa for NSPC mode.