A fault diagnosis based on bond graph model is proposed for hydraulic variable pitch system. Because the knowledge representation of bond graph model can provide information of cause and effect between components, a bond graph model of hydraulic variable pitch system is given above rated wind speed. A fault tree, using cause and effect analysis by back propagation, is developed. Qualitative value of parameters is assigned and the fault source is detected by analyzing boundary parameters of fault tree. Comparing with quantitative fault diagnosis based on model, the bond graph fault diagnosis is more flexible and has good completeness.