A laser processing apparatus includes worktable for loading a workpiece, a laser emitter, a user interface, a detecting unit, and a controller. The user interface allows inputting a thickness value of the workpiece. The detecting unit includes a signal emitter, a signal receiver for receiving signals emitted by the signal emitter, and a lifting unit. The workpiece is positioned between the signal emitter and the signal receiver. The controller controls the lifting device to adjust a height of the signal emitter and signal receiver relative to the worktable to be equal to a sum of the input thickness value and a focal length of the laser emitter. The controller controls the driving device to adjust a height of the laser emitter relative to the worktable if the signal receiver receives the signals and stop to adjust the laser emitter if the signal receiver does not receive the signals.