The best-possible-self (BPS) intervention is one of the most widely used positive psychological interventions. It has been shown to promote well-being, optimism and positive affect and to decrease negative affect. However, most research has been conducted in western countries and its effectiveness in people from Eastern countries has been questioned. In this study, we examined the effect of the BPS in young adults from mainland China. Moreover, we investigated the moderating effect of cultural orientation, self-efficacy, and trait optimism and the mediating effect of state optimism and goal-related cognitions. Seventy participants were randomly assigned to the BPS or the control condition. In contrast to previous studies, the BPS did not lead to higher positive affect or well-being in Chinese, but rather to decreased negative affect and negative future expectancies. No significant moderating or mediating effect was found. This study indicates that the effects of the BPS might differ across cultures.