Accurate identification and quantification of pores and microcracks in lightweight aggregate concrete (LAC) are important for analyzing salt frost deterioration mechanisms. Therefore, a new DIP method based on morphology principle was proposed, which overcomes the difficulties of traditional methods that can not accurately segment pores and microcracks because of only relying on shape factors, and the porous lightweight aggregate interferes with the analysis of pore and microcrack in concrete matrix. Using this method, the microstructure deterioration of shale ceramsite LAC and normal aggregate concrete (NAC) with same mortar to aggregate ratio and same water to cement ratio was compared and analyzed. With the increase of freeze-thaw cycles, the pore structure damage of the two concretes is manifested as pore expansion and microcrack initiation from the pore wall, leading to the relative dynamic modulus of elasticity (r_n) loss. Grey correlation analysis indicates that in addition to the initiation and propagation of microcracks, the high dispersion of LAC pore diameter also contributed to the loss of r_n in LAC. Test shows the new DIP method realizes the segmentation and independent analysis of pore and microcrack, and achieves the recognition accuracy to 10.6 μm, which has advantages in quantitative analysis of concrete pore structure and microcracks.