Growing evidence shows that positive psychology interventions can increase western people’s well-being. However, it is unclear whether individually-focused happiness interventions improve Chinese people’s well-being. Furthermore, factors that moderate the effect of happiness intervention strategies are yet to be explored. The study hypothesized that: Compared to the control activity, the two experimental activities will improve participants’ well-being and reduce their depression level; the person-activity fit moderates the effect of the happiness-enhancing activities; the effort participants devoted to a given activity moderates the effect of the activities. A longitudinal, placebo-controlled design was used, and 120 university students filled out Oxford Happiness Questionnaires, Positive and Negative Affect Schedule, Satisfaction With Life Scale, and Center for Epidemiological Studies Depression Scale. They were randomly assigned to express gratitude, cultivate optimism, and list everyday events (control group) and 104 students completed the assigned activities every week and persisted for 5 weeks. Then they completed the same scales as 5 weeks ago. Measures of person-activity fit and effort were also administered. After 5 weeks’ practices, gratitude group showed significant increase in overall well-being indicator (p = .008) and decrease in depression score (p < .001); optimism group showed significant decrease in depression score (p = .014); control group did not show significant change in well-being and depression (both ps > .1). In addition, gratitude group showed the biggest well-being increase relative to the other two groups (p = .029 and p = .011). Furthermore, the person-activity fit predicted post-intervention well-being (β = .170) and depression (β = -.198); the effort participants exerted in the activities predicted post-intervention depression (β = -.249). Finally, pre-intervention well-being negatively predicted well-being change (β = -.456), pre-intervention depression predicted depression change (β = .527), suggested that participants with the lowest well-being or highest depression level benefit most from the activities. In conclusion, relative to list everyday events, express gratitude can improve well-being and reduce depression, and cultivate optimism by imagining the ideal future can reduce depression. The participants who find the assigned activities fit themselves benefit more from the interventions, and they devote more effort to the activities. Participants with poor well-being benefit more from the interventions. The research provides valuable information about how to sustainably increase people’s happiness and the potential moderators.