Wearable devices have a rapid development and popularity in the last decade, and the human-computer interaction issues in this field have become prominent. In general, the screen size of wearable devices is much smaller. Besides, the usage scenario is usually outdoors under the state of moving. Consequently, it is difficult to apply the existing standards of other devices, such as laptops, on them. In this study, we used a smartwatch as the interaction device, and 45 users participated in this study. Two experiments were designed to investigate the influence of the key factors (font size, contrast, and weight) that influenced legibility of text (numbers and characters) on behavioral performance and subjective experience for two age groups (people aged 18–40 vs. people aged over 40). The results showed that the font size had the greatest influence on legibility. For people over 40, the very small size and the small size text could not be clearly recognized. For the young people aged 18–40, the font size had a subtle influence on performance. However, too large font size reduced the preference for message reading. Contrast is another variable that influence legibility. Low contrast needs to be avoided in the design. The influence of weight is relatively small, and very thin weight is worse than medium and very thick weight. In general, the method used in this study had high ecological validity and we hope to provide useful suggestions for the design of smartwatch.