The main purpose of this study was to explore the effect of positive and negative emotion on students' cognitive tasks switch cost, based on task-set reconfiguration and broadening hypothesizes. An alternating-runs paradigm experiment was conducted to compare switch cost among different conditions. 480 participants from elementary school, junior high school, high school, and college, assigned into three moods(positive emotion, negative emotion, and neutral mood) and two conditions (foreknowledge and no foreknowledge). Participants performed task repetition and task switch trails through six blocks. Tasks consisted of classification of figure or color, adding 5 or subtracting 3. And switch cost refers to the performance difference in task repetition and task switch trails. Analysis of variance was employed to test switch cost among different conditions, and compare switch cost of difference age and gender. Result demonstrated: (1) the time of task switch was significantly longer than that of task repetition; (2) switch cost under foreknowledge condition was smaller than that under no foreknowledge condition; (3)emotional conditions influenced switch cost, compared with neutral mood, positive emotion reduced switch cost, while negative emotion increased it; (4) there was no significantly difference between male and female' switch cost; (5) there was significantly different switch cost among elementary school, junior high school, high school, and college students. Based on those results, implication for switch cost reducing of cognition task and future research was discussed.