Objective To investigate the changes in dietary behavior and physical exercise of people of different ages during the epidemic of Coronavirus Disease 2019 (COVID-19), and to explore the influencing factors of weight gain. Methods A structured electronic questionnaire was used to investigate the basic conditions, weight, eating behavior and physical exercise of 1 125 subjects. The χ² test or non-parametric test was used to compare the changes in the three age groups (<30, 30~65, >65 years), and Logistic regression analysis was used to explore the factors affecting weight gain. Results Among the three age groups, young people under 30 years old changed greatly in the consumption of grains, red meat, and high-calorie snacks, and in the physical exercise, and there was a significant amount of people experiencing such changes, while for those older than 65 years old, the frequency of intake of grains, red meat, and high-calorie snacks before and after the outbreak was basically unchanged. In addition, compared with the population who basically did not change dietary behavior during the epidemic, the increase in the consumption of grains (OR = 2.469), red meat (OR = 1.582), and high-calorie snacks (OR = 1.815), and the decrease in physical exercise (OR = 1.702 ) were independent risk factors for weight gain during the epidemic (P <0.05). Conclusions During the epidemic of COVID-19, the young people (<30 years old) experienced the largest changes in dietary intake (grain, red meat, and high-calorie snacks) and exercise, while the elderly people (>65 years old) maintained basically unchanged. The increased intake of grains, red meat and high-calorie snacks, and lack of physical exercise could lead to weight gain.