The invention discloses a three-shaft automatic straightening machine special for a cam shaft and an automatic straightening method. The three-shaft automatic straightening machine comprises a tight pressing device and a bearing device. The tight pressing device presses a to-be-straightened cam shaft on the bearing device. The tight pressing device comprises a rack, a tight pressing oil cylinder, a guiding mechanism, a movable seat plate and an upper roller shaft. The bearing device is provided with a speed reducing motor, a first lower roller shaft and a second lower roller shaft, and both the first lower roller shaft and the second lower roller shaft can rotate. The axis of the upper roller shaft is located in the middle between the axis of the first lower roller shaft and the axis of the second lower roller shaft. The upper roller shaft, the first lower roller shaft and the second lower roller shaft are tangent to and abut against the to-be-straightened cam shaft. The three-shaft automatic straightening machine special for the cam shaft and the automatic straightening method can automatically straighten the cam shaft at a time and have the characteristics of being low in labor intensity and operation skill requirement and high in production efficiency.