This paper explored the main driving force of private placement discount based on the perspective of the investors’ sentiment. The results show that: (1) the private placement discount has been influenced by the dual effects of rational and irrational factors, but the irrational factors are the main driving force. Also, there is a significant positive correlation between investors’ sentiment and private placement discount: the more optimistic (pessimistic) the investors’ sentiment is, the greater (smaller) the private placement discounts are. (2) Short-term excessive optimism of investors led to the phenomenon of the long-term vulnerable private placement market.