In the process of drilling engineering construction， the mechanical failure will inevitably appear. The fault tree is one of the common methods to analyze the causes. But in the construction， due to the insufficient data or no attention to save these data， assigning value is difficult in the actual application and only qualitative analysis can be made. In view of the fuzziness and uncertainty when faults of the engineering construction machinery happened， the occurrence probability of the bottom event is described with fuzzy number， and a machinery fault tree analysis method for engineering construction based on triangular fuzzy number is proposed. This method uses the experience of experts， field technicians and operators to describe the probability interval of each bottom event. The median importance of the triangular fuzzy number is caculated and is used to sort the causes of the fault， which is served as the basis for formulating preventive measures. Finally， it is applied to the reason analysis of mix-material blocking pipe of long spiral drill， and makes targeted preventive measures for faults according to the fuzzy importance. The results show that the ranking of important influencing factors is consistent with the qualitative analysis and judgment of the field technicians and operators.