The calculation formula of equivalent reactance Xcon between generator and infinite system for generator out-of-step protection in the current " Guide of Calculating Settings of Relay Protection for Large Generator and Transformer " is improper, it is questionable. This paper has researched this problem deeply and obtained the correct method of the Xcon setting calculation for generator out-of-step protection, and compared the Xcon values calculated by the proposed calculation method and the " guidelines " calculation method in the same oscillation of the system condition, the results show that the "guidelines" setting calculation method may lead to the out-of-step protection maloperation, and the proposed setting calculation method can ensure the out-of-step protection reliable operation.