Abstract: In order to investigate the microscopic mechanism of thawing collapse and compression characteristics of soft clay, the thawing collapse tests, compression tests and mercury intrusion porosimetry (MIP) are adopted to study microscopic pore distribution and change of soft clay in Ningbo area during the period before and after freezing, thawing and compression. A method for pore diameter division of four kinds of soil samples is proposed, including undisturbed soil, melted soil, compressed undisturbed soil and compressed thawing soil. Through scanning electron microscopy and mercury intrusion tests, the microstructural changes of freeze-thaw soft clay are analyzed according to the fractal theory of mercury. The characteristics of pore diameter distribution of soft clay are investigated, and the pore volume and area distribution are calculated by the capacity dimension. The results show that the pore volume and area of soft clay both increase after melting. The pore volume and area of undisturbed soil and thawing soil both decrease after compression, and more than 80% of pore volume and area of soft clay are distributed in the micro pores and ultra-micro pores. Besides, fractal characteristics exist in the pore volume and area.