The study aimed to investigate the direct effect of volunteer motivation on social problem-solving ability, self-efficacy, and mental health and examine the mediating effects on volunteer motivation and mental health by testing social problem-solving ability and self-efficacy. We examined a hypothetical model that integrates the concepts of Self-Determination Theory, the Social Problem Solving Model, and the Self-Efficacy Belief Model. Results demonstrated that volunteer motivation was significantly associated with social problem-solving ability and self-efficacy amongst 1530 undergraduate students. Three subscales of social problem-solving ability (positive problem orientation, negative problem orientation, and avoidance style) and self-efficacy served are significant mediators. Model statistics for SEM demonstrated an adequate fit with the data. Volunteer motivation provides a way to enhance social problem-solving ability and self-efficacy. It also leads to better mental health by enhancing positive problem orientation and self-efficacy and by reducing negative problem orientation and avoidance style.