The relationship between hot diffusion aluminizing temperature, infiltration time and thickness of aluminized coating of X70 pipe line steel was researched by quadratic orthogonal regression testing method. The regression equation of the alloying layer thickness was established. The optimum process for hot diffusion aluminizing treatment was gotten. The surface and interface morphologies of X70 pipeline steel after the aluminizing treatment were observed, and Fe and Al content change by the carbonitride layer were analyzed. The element diffusion mechanism of hot dipping alumetizing process was discussed. The results show that alloying layer thickness is increased with the rising of insulation temperture and the extending of diffusion time. The ideal inoulation temperature is 950 ℃ and time is insulation 6 h.