Diaries have been widely used among people with dementia as a data collection tool and memory aid. An understanding of their usage is conducive to the subsequent development of intervention measures, but comprehensive evidence is still lacking. This review synthesises the existing literature to obtain an in-depth understanding of the positive and negative effects of diary usage among people with dementia and the influencing factors. The data were collected from eight databases from the inception of each database to November 2020. A thematic synthesis method was used to sort and summarise the included studies. Ten studies were chosen for the review. The following three analytical themes were identified: (i) positive changes; (ii) factors affecting the record; and (iii) negative feelings. Diaries can cause positive changes in people with dementia, but the specific patterns of stimulation and changes over time remain to be explored. Furthermore, researchers should encourage patients and their families to have a correct understanding of the diary, improve their motivation and social support, fully use the positive effects of the diary, and reduce the occurrence of negative effects. Medical staff should help caregivers improve their awareness of the usefulness and ease of the diary and encourage them to provide support and companionship to people with dementia while exhibiting a positive attitude.