The invention discloses an endurance test device of idle motors of an automobile. The endurance test device comprises a motor positioning plate (2) and a fixture positioning plate (3) used for installing test parts of the endurance test device. An output shaft of the idle motor (1) extends downwards, the extending end is provided with a sealing cap (13), a cushion block (11) is arranged between the motor positioning plate (2) and the fixture positioning plate (3), and the motor positioning plate (2) is fixedly connected with the fixture positioning plate (3) by the cushion block (11). The invention also discloses an adjusting method of the endurance test device. By adopting the technical scheme, a plurality of fixtures can be installed simultaneously, the endurance test can be carried out at the same time, and adjustment can be carried out on different conditions that the motors have different types or the motors have the same types, but the sealing caps of the top end of the motors are different. When a plurality of idle motors exists, the adjusting method only needs to adjust the endurance test device once so as to meet the requirement of all of the motors. Stepless adjustment can be carried out on thet test stroke of the motor and the maximum of the rated load of the motor.