Brake drag is the main factor affecting the transmission efficiency and vehicle fuel consumption. This paper focused on analyzing the impact of assembled disc lateral runout on brake drag. First the impact mechanism of lateral disc runout on drag was analyzed theoretically. Then the brake drag torque under different assembled disc lateral runout was tested to figure out the relationship between them. And then,the influence factors on disc lateral runout were analyzed and the disc lateral runout was optimized. Finally,the vehicle resistance of the original car and the prototype with optimized brake were compared. The result shows that the vehicle resistance after optimized is reduced by 3%. Brake drag is the main factor affecting the transmission efficiency and vehicle fuel consumption. This paper focused on analyzing the impact of assembled disc lateral runout on brake drag. First the impact mechanism of lateral disc runout on drag was analyzed theoretically. Then the brake drag torque under different assembled disc lateral runout was tested to figure out the relationship between them. And then, the influence factors on disc lateral runout were optimized. brake were compared. The result shows that the vehicle resistance after optimized is reduced by 3%.