Based on the research of electromagnetic brake, a mechanics model of the towed trailer with electromagnetic brake was established in this paper, according to the braking rules about the braking force of towed trailer. The relationship between the braking force and the current of electromagnet by virtue of the braking efficiency method was acquired. And the proper scope of the current is obtained, A comparative analysis was made between the conclusion and the test results. It proves the correspondence of the practice with the conclusion, which verifies the mechanical model of the braking force and provides valuable basis for the further study on the distribution of the braking force and the braking stability.