The water retention and anti-cracking ability of clay soils improved by lignin-derived polymer are studied by indoor evaporation tests. To explore the crack evolution patterns during the wet-dry cycle, the effects of different contents of polymer on internal moisture variation in improved soils were analyzed and the quantitative analysis was carried out for crack network on the soil surface using relevant image analysis software. The results show that: (1) The water retention capacity of improved soil is initially enhanced and then declined with increasing polymer content. When the content is 1.0%, the duration of the constant evaporation rate stage is extended by about 4 hours compared to the untreated soil and the average evaporation rate of this stage is reduced by about 22.95%. (2) The development of surface cracks is effectively limited by aggregating soil particles and water. However, excessive addition of polymer causes local stress concentration and induces extensive crack development. (3) The moderate content of polymer suppresses the development of cracks and the formation of wide cracks, thus enhances the "healing" capability of crack network during the wet-dry cycle.