We advance an improved three implication reasoning algorithm based on RM arithmetic operators.As for FMP(fuzzy modus pronens) problem,we provide the solution for multidimensional,multiple and multidimensional and multiple rules with this algorithm.And we theoretically analyze its compute performance,and think it more excellent than general three implications reasoning one.In the end,on the basis of analyzing the characters of fish disease diagnosis and communication with fish disease expert,we extract the rules for fish disease diagnosis,put forward a general model for fish disease fuzzy diagnosis and show up our algorithm for this model.In our algorithm fuzzy logical reasoning that we used is just the three implication reasoning based on RM arithmetical operators we advanced.