Fear of missing out and passive social networking site use have become widespread phenomena among college students, which would bring a series of detrimental outcomes, such as poor sleep, cognitive failures, decreased life satisfaction, and externalizing problems. Recently, some empirical studies have discussed the relationship between these two constructs among college students. However, the results based on cross-sectional designs are far from consistent. One group of researchers believed that fear of missing out might be a risk factor for the passive social networking site use based on the “Uses and Gratification Theory”, another group of researchers suggested that fear of missing out might be triggered by the passive social networking site use on the basis of “Replacement Hypothesis”, still other researchers hold that there may be a bidirectional association between fear of missing out and passive social networking site use based on “Reinforcing Spirals Model”. Therefore the direction of the relationship between these two constructs remains unclear, and longitudinal data is needed to clarify this issue. This study used a two-wave cross-lagged panel analysis to explore the reciprocal relationship between fear of missing out and passive social networking site use, and the moderating role of gender. In our study, four hundred and three Chinese college students completed questionnaires including fear of missing out scale and passive social networking site use scale twice with an interval of eight months. The data are analyzed using SPSS22.0 and AMOS23.0, including repeated measurement of variance analysis, correlation analysis and path analysis of the cross-lagged model. By testing autoregressive effects using longitudinal data, we found that fear of missing out and passive social networking site use in college students at an earlier time were important predictors for the same conditions at a later time. Moreover, the cross-lagged analyses revealed that the relationship between fear of missing out and passive social networking site use was bidirectional over the eight months. Additionally, the multiple-group analysis showed that there was no significant gender difference of the cross-lagged model. The findings of this study supported the view of the reinforcing spirals model, which indicated that higher level of fear of missing out predicts more frequent passive social networking site use over time and vice versa. The result reminds us that timely psychological counseling is needed even for college students who feel mild fear of missing out because it might lead to severe fear of missing out over time. In addition, we should also focus on educating college students about the proper usage of social network sites from an earlier age in order to prevent more frequent passive social networking site use in advance. More importantly, we should be aware that for both boys and girls, reducing fear of missing out is beneficial to the prevention of serious passive social networking site use and vice versa.