Unauthorized Mainland Chinese and other foreign boats entering the restricted and forbidden maritime area have always been a threat to this nation. To maintain the security and order of this nation, expulsion of trespassing boats has been the goal of the maritime law enforcement agency. In order to ensure the expulsion, the relevant countermeasures should be adequately planned in advance. In the planning process the future number of trespassing boats is an important referential index. This study applies the SARIMA model based on the monthly data from 2000 to 2006 to forecast the future number of expulsions. In comparison with the traditional regression model, the explanative and forecast ability of the SARIMA model is more effective which could provide the reference for maritime law enforcement planning.