This research aims to examine the relationships among spontaneity, psychological capital and mental health. The research used a questionnaire, including demographic data and four scales: spontaneity, psychological capital, happiness and common mental disorders. Using purposive sampling with two universities in Midwest Taiwan, the study collected 516 valid questionnaires. Using structural equation modeling analysis, the results showed that spontaneity enhanced psychological capital, which in turn improved mental health. Specifically, spontaneity including five factors (free, pleasure, confidence, love and vigor) enhanced psychological capital, which in turn increased happiness and alleviated common mental disorders. Spontaneity and psychological capital are vital sources of university students' mental health. The researchers presented recommendations and limitations.