For solving the problem of the abandonment of the career in nursing undergraduates, it is important to understand their motivation to choose nursing as a career and its associated personal and situational factors. To examine the relationships between optimism, educational environment, career adaptability, and career motivation in nursing undergraduates using the career construction model of adaptation. This study adopted a cross-sectional design. A convenience sample of 1060 nursing undergraduates from three universities completed questionnaires for measuring optimism, educational environment, career adaptability, and career motivation. Confirmatory factor analyses, descriptive analyses, comparison analyses, correlation analyses, and mediation analyses were performed accordingly. Nursing undergraduates' career motivation was positively correlated with their career adaptability (r = 0.41, P < 0.01), the educational environment (r = 0.60, P < 0.01), and optimism (r = 0.26, P < 0.01). In addition, the effects of optimism and educational environment on career motivation were partially mediated by career adaptability in nursing undergraduates. In nursing undergraduates, the educational environment had a relatively strong positive association with career motivation, while optimism had a weak one. Career adaptability played a mediating role in the relationships. Targeted interventions may improve nursing undergraduates' career motivation.