A data packet transmission method and system, a terminal device and a network device are disclosed. In the method, a terminal device acquires a condition on which a small data packet transmission mechanism is used; and if the condition is satisfied, the terminal device transmits a data packet according to the small data transmission mechanism, where the condition includes any one or more of the following: a data amount of the data packet is less than a first threshold; transmission of the data packet is within a pre-configured time; within a sliding time window, a total data amount of data packets that have been transmitted by the terminal device by using the small data packet transmission mechanism does not exceed a second threshold; an application class of the data packet is a pre-configured application class; and the terminal device is of a pre-configured access class.