In order to reduce the faults and enormous economic losses of mining excavator's elevating controlling system,the research of the system's fault diagnostic methods is very valuable. Based on functional fault description and fault arrangement, a new method is offered for the complicated controlling system . The improved fuzzy algorithm of the controlling fields is introduced into the fault diagnostic fields. A improved fuzzy diagnostic model for the voltage's minus-feedback circuit of WK-4 excavator's elevating controlling system is built up and compared in detail with the traditional fuzzy diagnostic model. The results show the effectiveness and advantage of the improved fuzzy diagnostic model. The method proposed can be widely applied to fault diagnosis of mining excavator's various controlling systems.