Some studies suggest that fund investment is a sound strategy, but ordinary investors often prefer to make their own buying and selling decisions rather than investing in funds. Previous studies have shown that in the face of complex fund investment choices, investors will use different information processing methods and will also be affected by external environment. Therefore, based on the self-determination theory, this study divided the influencing factors of fund investment into internal motivation factors (motivation types, intertemporal decision making, future time insight) and a external motivation factor (social comparison). The study comprised two sub-studies: Research 1 investigated how internal motivation impacted fund investment choice; Research 2 examined how both internal and external motivation factors influenced fund investment choice when they coexist, as well as whether fund investment choice behavior is self-determined or non-self-determined. The results revealed that: (1) The main effects of motivation type and future time insight were significant. Intertemporal decision making interacted significantly with present fatalistic views of time. (2) Social comparison interact significantly with future time insight and intertemporal decision making respectively. (3) The interaction between motivation type, intertemporal decision making, and future time insight was found to be significant. In conclusion, external motivation alone could not independently affect fund investment behavior. In conclusion, fund investment is a self-determined behavior primarily influenced by internal motivation.