The invention discloses a plasma beam and ultrasonic wave combination device and a method for preparing a crack-free expansion cone through the plasma beam and ultrasonic wave combination device. Thedevice comprises a plasma beam surface treating machine, an ultrasonic vibrator arranged on a sample mounting shaft of the plasma beam surface treating machine and an ultrasonic generator connected with the ultrasonic vibrator. The method for preparing the crack-free expansion cone through the plasma beam and ultrasonic wave combination device comprises the following steps of (1) preparing evenly-mixed iron-based alloy powder for inner, middle and outer alloy layers; (2) conducting plasma surface treatment on a crack-free expansion cone blank to be treated through the plasma beam and ultrasonic wave combination device under the condition of ultrasonic vibration to form inner, middle and outer alloy protective layers, wherein each protective layer needs to be subjected to cooling and fine grinding, and then the next protective layer is prepared. The method integrates ultrasonic vibration and plasma beam surface metallurgy, the fine-grained crack-free alloy protective layers with the compact structure and low porosity are prepared on the surface of the expansion cone, the abrasion resistance of the expansion cone can be remarkably improved, and the service life of the expansion conecan be greatly prolonged.