The invention provides a network address conversion method and device. The method comprises the steps that a first message which is sent by an intranet user and is used for initiating connection to a public network device is received; a corresponding first public network IP address and port block are assigned to the source IP address of the first message; if the assigned port block is not split, the port block is split into at least two small port blocks; each small port block is assigned to a corresponding service board according to a preset assigning rule; the status of each small port block assigned to the corresponding service board is active, so that each service board determines the port block statistic usage amount according to the status of the small port block; and the first public network IP address and a public network port assigned to a local small port block are used to carry out network address translation on the first message. According to the application, the situation that the port block statistic usage amount of a certain service board is greater than the port block actual usage amount can be avoided, and the port block utilization rate is improved.