To compare the cognitive function and mental behavior between Alzheimer's disease (AD) and Parkinson's disease (PD). Methods: Fifty-two patients with AD (AD group) and 32 patients with PD (PDD group) in our hospital were enrolled. The cognitive function and mental behavior of the two groups were evaluated by relevant assessments (CDR, MMSE, MoCA, CDT, and CNPI). The cognitive function and mental behavior characteristics of each group were summarized. Results: There was no significant difference in dementia severity and in MMSE and MoCA scores between the two groups (P>0.05). Compared with that of the AD group, CDT score of the PDD group was significantly lower and CNPI score was significantly higher (P< 0.05). There were no significant difference in naming, attention, language, abstract thinking, and orientation between the AD group and PDD group (P>0.05). Compared with the PDD group, the AD group had higher scores in visuospatial/executive function and lower scores in memory and delayed memory dimensions (P< 0.05). There were no significant difference between the two groups in the incidence of delusion, euphoria, inhibition, abnormal movement behavior, nighttime sleep behavior, appetite, and eating disorders (P>0.05). Compared with that in the AD group, the incidence of hallucination, agitation, and depression in the PDD group was higher, and the incidence of anxiety, indifference, and irritability were lower (P<0.05). Conclusion: Compared to AD, PDD shows greater impairment of executive function and visuospatial function. The mental behavior symptoms of PDD are hallucination, agitation, and depression, while those of AD are anxiety, indifference, and irritability.