As declining birth rates emerge as a significant societal concern, understanding the implications of being an only child versus having siblings is becoming increasingly important in China, especially in relation to social and emotional development. This longitudinal study examines the role of solitude preference and sibling status in the relationships between negative emotionality and loneliness in early childhood. The participants were 204 children (Mage= 48.58 months, SD = 3.60). At Time 1, the children were interviewed to assess their preference for solitary play and loneliness, with a follow-up assessment of loneliness conducted at Time 2, two years later. Meanwhile, mothers provided evaluations of their children's negative emotionality at Time 1. The findings indicate that negative emotionality positively predicts loneliness two years later, with solitude preference acting as a mediator in this relationship. Additionally, sibling status was found to moderate the link between negative emotionality and solitude preference. Specifically, for only children, higher negative emotionality intensified the relationship between solitude preference and loneliness, whereas having siblings acted as a protective factor, mitigating this effect. These results underscore the potential social and psychological disadvantages faced by only children and highlight the protective role siblings can play in mitigating feelings of loneliness.