Abstract: Post-grouting steel pipe micropiles are widely used in the underpinning and soft ground reinforcement engineering due to its fast construction, good adaptability and high bearing capacity. However, there is little research on the grouting diffusion mechanisms and the bearing capacity of the micropiles in marine muddy soil. Field tests on the bearing capacity of both grouted and non-grouted steel pipe micropiles in muddy soil in Lianyungang were carried out. The influence of post-grouting on the bearing capacity of the steel pipe micropiles was evaluated. Based on the transparent clay and the particle image velocimetry (PIV) technique, visual model tests on the construction process of steel pipe micropiles were conducted. The displacement of surrounding soil and side-grouting diffusion in transparent clay were measured and discussed. Field test results show that the bearing capacity of the grouted micropile was improved to nearly 2.4 times compared to that of the non-grouted micropiles. It also shows that the maximum influence range of the pile installation in the horizontal direction was about 4.5D from the center of the micropile; and grouting diffusion in the muddy soil mainly belonged to fracture grouting in the horizontal direction.