The invention provides an automatic error measuring device for a digital controlled lathe and comprises a lever-type inductance position finder, a step gauge, a clamp, a clamping mechanism as well as a data collection and process device. The measuring joint of the lever-type inductance position finder is contacted with the measuring datum plane of the step gauge; the clamping end of the lever-type inductance position finder is connected with the clamping end of the clamping mechanism; the lever-type inductance position finder is connected with the data collection and process device to transmit a displacement signal to the data collection and process device; the step gauge is arranged on the clamp. The invention improves the error measuring efficiency of the digital controlled lathe and reduces the complexity of error measuring operation and the error measuring cost.