Background: Understanding undergraduates’ dietary literacy, including dietary knowledge, attitude and practice (KAP), is important for future health promotion policies. Therefore, this study aimed to investigate the current status and influencing factors of dietary literacy in Chinese college undergraduates and explore whether a nutritional lecture could improve their dietary literacy. Methods: This study included two parts: a cross-sectional study (n = 1026) conducted by a dietary literacy questionnaire, and a randomized controlled trail (RCT) that enrolled 99 college undergraduates who were randomized to a control group or a nutritional lecture group. Data from the questionnaire and 72 h food records were obtained on day 0, day 3 and day 100 before and after intervention. Results: This cross-sectional study showed that the mean scores of dietary KAP were respectively 100.0 (33.3), 59.1 (13.6) and 71.7 (11.7), with an excellent rate of 36.6%, 1.9% and 3.4%. Female upper-grade undergraduates and those with medicine-related majors illustrated higher dietary knowledge scores (p < 0.001). Dietary attitude rather than dietary knowledge had a significant impact on dietary practice (p < 0.001). This finding was in line with the results in the RCT study. Compared with the control group, dietary knowledge was significantly improved in the nutritional lecture group on both day 3 (p = 0.002) and day 100 (p = 0.023) after intervention. However, dietary behavior was improved only on day 3 post nutritional lecture (p = 0.029) but decreased to the original level 100 days later (p < 0.001). Conclusions: This study discovered the unsatisfying status and discrepancy between dietary literacy among college undergraduates of different sex, majors and college years. Dietary attitude instead of dietary knowledge was discovered as a determining role in dietary practice. One nutritional lecture could improve undergraduates’ dietary literacy but the effect was not long-lasting. Further studies with more reinforced and durable interventions are warranted.