Select-in and select-out are two completely different strategies of military psychological selection. The two strategies vary from each other theoretically and practically. Select-in is a process of predicting job performance based on person-job fit theory and competency theory. Psychological selection of military officers in Germany is a typical select-in selection. Select-out is actually a kind of physiological and psychological screening. Select-out psychological selection aims at screening out retarded or mentally ill candidates. Psychological selection for Chinese recruits is a typical select-out selection. Select-in and select-out are different in test items and test administration. Three factors should be taken into account in the trade-off between select-in and select-out:selection objects, number of candidates and admission ratio, and cost-benefit ratio. Select-in and select-out can be merged perfectly, which can be seen in Subscreen of US submarine. In practice, selection objects, number of candidates to admission ratio, and cost-benefit ratio suggest that the select-in strategy should be taken as priority. The combination of select-in and select-out can also be used in the large-scale personnel selection in the army.