The treatment of peat soil foundation in Yunnan surrounding Dianchi Lake and Erhai Lake poses more and more complex problems for actual engineering projects. It is no longer enough to rely on ordinary cement to reinforce peat soil. This experiment prepares cement-soil samples by mixing humic acid (HA), cement, and ultrafine cement (UFC) into the low organic matter alluvial cohesive soil (blending method). Then, the test method of soaking it in fulvic acid (FA) is used to simulate the cement-soil in the peat soil environment (steeping method). This study performed unconfined compressive strength (UCS) and scanning electron microscope (SEM) tests on samples soaked for 28 days and 90 days, analyzing the influence of UFC proportion on the strength of cement-soil in the peat soil environment. The test results are as follows. Without considering the effects of HA and FA, incorporating UFC can significantly improve the UCS of cement-soil. The rapid hydration of the fine particles to generate a large number of cementitious products improves the cohesion of the soil skeleton and fills the pores. However, when the proportion of UFC increases, the aggregate structure formed by too many fine particles reduces the hydration rate and degree of cement hydration, making the UCS growth rate of cement-soil insignificant. In the peat soil environment, HA significantly weakened the UCS of cement-soil in both physical and chemical aspects. However, UFC can weaken the adverse effect of HA on cement-soil by its small particle size, high surface energy, and solid binding ability. In addition, FA has a certain enhancement effect on the UCS of cement-soil soaked for 28d and 90d, and the addition of UFC can appropriately promote the enhancement effect of FA on cement-soil UCS. SEM test results showed that cement hydration products increased significantly with the increase of UFC proportion. The cementation between hydration products and soil particles is significantly enhanced. The size and connectivity of cement-soil pores are significantly reduced, and the structural integrity is enhanced.