The determination of reasonable construction period is of great significance for the  instruction and control of the project construction. However, since reasonable period in itself is of relativity and fuzziness, how to identify and assess a reasonable construction period is complex. The construction period of diversion tunnel will be affected by many factors like construction scale, excavation methods, surrounding rock type and distribution, and construction equipment configuration, among which some factors are uncertain. We present a method of forecasting the construction period of diversion tunnel based on project cases analogy and causal clustering fuzzy prediction. We applied the method to the prediction of ten diversion channels of three completed projects, and the prediction results are very close to or even coincide with the actual construction period, which verified the feasibility of the method.   