Online health information service has increasingly become an effective means for health management, and is widely concerned by people from all walks of life. In order to solve the problem of dynamic pricing and promotion strategy of online health information service, this paper considers a kind of online health information service provider, which simultaneously provides customized health information service and free public health information service. With the goal of maximizing the profit of online health information service providers, based on the differential dynamics theory, this paper analyzes the dynamic conversion characteristics between consumers of free public health information and customized health information service, and constructs the optimal control model of dynamic pricing mechanism for health information service. The results show that for online health information service with long service period, giving consumers a certain period of ‘free-charge period’ before charging service can increase the optimal profit of online information service providers. However for online health service with short service period, it is not necessary to set the ‘free-charge period’. Also, both the growth rate of potential customers of online health information service providers and the length of service time have significant impacts on the degree of public health information delivery. At the initial stage of online health information service, the service providers can get more profits by appropriately extending the ‘free-charge period’ for high-income consumers. All these conclusions are valuable for health information service providers to make service pricing and promotion strategies.