When using the draft-tube vacuum to be less than 8.0m as the rule to set a tailrace surge tank, a mixing function that describes the process of water-hammer vacuum and velocity-head vacuum varied with time is proposed, on the assumption that the guide vane of the hydraulic turbine and the turbine discharge were all changed linearly. An exact maximum of the draft-tube vacuum for the first-phase water-hammer and the last-phase water-hammer is obtained. Finally a much more reasonable formula of critical tailrace length is derived. The results of two cases show that the formula proposed can determine correctly and reasonably whether a tailrace surge tank is needed or not, and are more suitable for project design than the formula suggested by the specification.