The main testing parameter of the roller brake tester is the braking force. Actually, there are some differences in results even if the same vehicle is tested on the same tester. So it will bring trouble to evaluate the braking performance accurately. Based on force analysis, the mathematical model of the roller opposite force type automobile brake tester is built in this article. And then the factors of influencing braking force value will be analyzed by theoretical calculations Taking the instance of the Jianghuai light truck and using the mathematical model analyze influencing factors for testing results and calculate error value. The results showed that adhesion cofficients of between testing wheels and rollers and between non-testing wheels and floor, structure of the roller brake testor have an influece on testing results of braking force and structure of the roller brake testor is the least. The theoretical analysis provide references for comparison test among testing equipments of the braking performance.