Treatment of peat soil foundation in Yunnan surrounding Dianchi and Erhai Lakes poses complex problems for engineering projects. It is insufficient to rely on ordinary cement to reinforce peat soil. In order to make the reinforcement reliable, this experiment mixed (ultrafine cement) UFC into ordinary cement to form a composite solidify agent. This study aimed to analyze the influence of UFC proportion on the strength of cement-soil in the peat soil environment. Unconfined compressive strength (UCS) and scanning electron microscope (SEM) tests were conducted on samples soaked for 28 and 90 days, respectively. The test results show that without considering the effects of Humic Acid (HA) and Fulvic Acid (FA), incorporating UFC can significantly improve the UCS of cement-soil. The rapid hydration of the fine particles generates 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 a larger quantity of 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 mitigate 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 positive effect on the UCS of cement-soil soaked for 28 days and 90 days. The UFC addition could 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, and cementation between hydration products and soil particles was enhanced. The size and connectivity of cement-soil pores were significantly reduced, thereby improving cement-soil structural integrity.