Background: The online health-care community (OHC) is not just a place for the public to share physician reviews or medical knowledge; it is also a physician-patient communication platform. The medical resources of developing countries are relatively inadequate, and the OHC is a potential solution to alleviate the phenomenon of long hospital queues and the lack of medical resources in rural areas. However, the success of the OHC depends on online contributions by physicians. Objective: The aim of this study is not only to understand physician online contribution behavior, but also to examine the helpfulness of incentive mechanisms. We addressed the following questions: (1) From which specialty area are physicians more likely to participate in OHC activities? (2) What are the factors affecting physician online contributions? (3) Do incentive mechanisms, including psychological and material rewards, result in differences of physician online contributions? (4) How can OHC managers exploit feasible incentive mechanisms to improve physician online contributions? Methods: We designed a longitudinal study involving a data sample in three waves. All data were collected from the Good Doctor website, which is the largest OHC in China. We first used descriptive statistics to investigate all of the physician online contribution behavior. Then multiple linear and quadratic regression models were applied to verify the causal relationship between rewards and physician online contribution. Results: Our sample includes 40,300 physicians from 3,607 different hospitals, 10 different major specialty areas, and 31 different provinces or municipalities. The average increments of physician online contribution scores in Gynecology-Obstetrics and Pediatrics specialty areas were 413.5 and 362.2, respectively, much higher than others. Based on the multiple quadratic regression model, we found that the coefficients of the control variables, past physician online contributions, doctor review rating, clinic title, hospital level, and city level, were 0.415, 0.189, -0.099, -0.106, and -0.143, respectively. For the psychological (material) rewards, the standardized coefficient of the main effect was 0.261 (0.688) and the standardized coefficient of the quadratic effect was -0.015 (-0.049). All estimates were statistically significantly at a 0.1% level. Conclusions: Our study discovered that the averages of physician online contribution were different in ten specialty areas. Physicians with more past physician online contribution, with higher review ratings, with lower clinic levels, not coming from tertiary hospitals, and not coming from big cities, were more willing to participate in OHC activities. To promote physician online contribution, it is necessary to establish an appropriate incentive mechanism including psychological and material rewards. Finally, our findings suggest two guidelines for designing a useful incentive mechanism to facilitate physician online contribution. First, material reward is more useful than psychological...