Past studies on aptitude for interpreting have investigated the correlation between personal traits and achievement, yet relatively few have singled out students' emotional state as an important aptitude constituent or tracked its developmental patterns. To address this gap, the present study followed 116 Chinese students of interpreting for five months (one semester) and examined their emotional reports as well as interpreting proficiency. The study differentiated learner-internal (positive and negative emotional traits) from learner-external emotional aspects (positive and negative emotional experiences), then administered questionnaires at three time points (the 1st month, the 3rd month, the 5th month) to determine the emotion-proficiency relations. Based on the regression analysis of the questionnaire data and qualitative analysis of students' emotion reports, the study found that: (1) As students' proficiency matured, they experienced growing positive and negative external emotions yet their internal emotions remained relatively stable; (2) While students' internal emotions were only moderately related to proficiency, their positive emotional experience was always the most reliable proficiency indicator; (3) Positive external emotions could stimulate students' long-term engagement with learning, and negative emotions such as anxiety could also promote learning by prompting students to use anxiety-reduction strategies. Specific suggestions are thus formulated for interpreter education.