Objective To investigate the effect of β-amyloid (Aβ) deposition on cognitive function in patients with mild cognitive impairment (MCI). Methods A total of 37 MCI patients admitted in our hospital from June 2014 to June 2017 were enrolled in the study. They were 15 males and 22 females, and aged (70.51±6.57) years. ¹¹C-Pittsburgh compound B (¹¹C-PIB) positron emission tomography/computed tomography (PET/CT) was performed to calculate the standardized uptake value ratio (SUVR). With 1.5 as the cut-off value, these patients were divided into Aβ deposition (Aβ⁺) and non-deposition (Aβ-) groups. Neuropsychological assessment and plasma Aβ level test were carried out on all subjects. SPSS statistics 18.0 was used to analyze the data. Results The patients of Aβ⁺ group had poorer general cognitive ability, memory function and executive function. The SUVR obtained by ¹¹C-PIB PET-CT was negatively correlated with the score of mini-mental state examination (MMSE) and memory function (r=-0.362, P<0.05; r=-0.403, P<0.05), but had no association with executive function, visuospatial ability or language function (P>0.05). Linear regression analysis showed that Aβ deposition exerted its effect on cognitive function in aspects of general cognitive ability and memory function (P<0.05). Conclusion Aβ deposition is associated with cognitive decline in MCI patients.