Studies have demonstrated that a forest experience can improve the mental state of visitors. The questionnaire methodology accounts for most of current findings, but subjective bias in self-reported scores cannot be eliminated. In this study, FireFACE V3.0 software was used to evaluate facial expressions of visitors unconsciously entering (urban experiencer) or leaving an urban forest park (forest experiencer) from 9:00 am to 5:00 pm. A total of 458 photos were tested for the hit-rate of machine-recognized facial expression scores to those identified by experts. Neutral, happy, sad, and scared expressions passed the validation test and were used for further analysis. The positive response index (PRI) was calculated as the difference between happy and sad scores. All five variables were analyzed by a mixed-model analysis of variance of walking direction (into the park vs. out of the park), gender (male vs. female), and age (senior, middle-aged, and young) on repeated records every 30 min in a daytime except noon. In the morning, pedestrians walking out of the forest park did not show more positive expressions than those leaving the park. Young visitors walking through the exit averaged a higher sadness score but a lower PRI than middle-aged visitors. In the afternoon, middle-aged visitors walking through the entrance averaged a higher PRI relative to young visitors leaving the park in the morning. Compared to urban experiencers, forest experiencers averaged lower scores for neutral and scared expressions, but forest experiencers unlikely revealed more positive expressions. Therefore, a forest experience does not always evoke positive expression on unconscious faces which was a responsive variable to go with time. Middle-aged people are encouraged to walk in forests with a longer time than youths in the morning. Male visitors, especially those in senior ages, are suggested to spend more time in forests than females.