Provided are a transmission parameter determination method, a terminal device and a network device. The method comprises: the terminal device determines a first transmission parameter according to the channel busy ratio (CBR) and the priority of service to be transmitted; and the terminal device sends the service to be transmitted using the first transmission parameter, and further comprises: the terminal device acquires at least one proportion coefficient, the at least one proportion coefficient being configured by a network device or pre-configured in the terminal device, wherein the operation of determining the first transmission parameter according to the CBR and the priority of the service to be transmitted comprises: the terminal device determines a second transmission parameter according to the CBR and the priority of the service to be transmitted, and processes the second transmission parameter using the at least one proportion coefficient to obtain the first transmission parameter.