For promoting a green, low-carbon way of disposition of architecture sediment and resource utilization, alkali-activated blast furnace slag (GGBS) as a green solidification agent was adopted to solidify mucky engineering waste soil for subgrade filling. The influence of different dosages of solidification agent on the comprehensive performance of solidified soil was explored. Furthermore, X-ray diffraction, scanning electron microscopy and nuclear magnetic resonance were carried out to characterize the properties of the solidification. Fractal theory was used to describe the micropore characteristics and a generalized model was established to reveal the mechanism of alkali-activated GGBS solidification of muddy muck. The results show that the liquid limit and plastic limit of the solidified soil is increased and the plasticity index is reduced owing to the curing agent. The optimum moisture content decreased first and then increased with the increase of the content of the curing agent. The maximum dry density is the opposite compared with that of the optimum moisture content. The addition of curing agent can improve the mechanical properties and water stability of slag soil. The strength, CBR, water stability, and permeability of 10% solidified slag is 2.36 MPa, 210.7%, 81.3%, and 1.36 × 10 ^-7 cm/s, respectively. All indicators can satisfy the engineering application requirement. The micro-mechanism showes that the main hydration products of the improved soil are (N, C)-A-S-H and ettringite Aft. The number of main pores in the solidified soil is reduced. For the slag with 10% of calcium solidification agent, the middle pores are reduced by 14.7%, and the fractal dimension of pore distribution is reduced from 1.23 to 1.06. The solidification of muddy engineering waste soil by alkali activated GGBS has improved the stability of the waste soil and has good application prospects.