The invention discloses an automatic horizontal adjusting device and adjusting method for platform. The automatic horizontal adjusting device for the platform comprises a bedplate, wherein an air spring is mounted under each corner of the bedplate, the air springs are connected with the upper ends of lifting mechanisms mounted in support legs, the lower ends of the lifting mechanisms are connected with transmission devices, a base is arranged under the support legs, a support seat A is mounted in the middle position under the bedplate, adjusting gas valves are also fixedly mounted under the bedplate, the adjusting gas valves are connected with the air springs through gas pipes, one end of a swing rod is provided with a spherical head A and is fixed by a clamp sleeve sleeved on the exterior of the support seat A, the other end of the swing rod is provided with a plumb bob, a suspension arm is mounted on the rod body of the swing rod, the end head of the suspension arm is correspondingly positioned under contact points of the adjusting gas valves, a locking plate is fixedly arranged on the base, and a movable magnet block inside the plumb bob controls the locking through a remote control switch. The invention also provides the horizontal adjusting method for the platform, and the horizontal adjusting device and the adjusting method have the characteristics that the horizontal adjusting time is short, the horizontal adjusting accuracy is high, the reliability is high and the like.