In the practical power system operation， the powers of generators and loads are changing constantly， and the state variables（e.g. power angle， rotating speed） and algebraic variables（e.g. voltages and currents of dq -axis） are fluctuant within a certain range. Probabilistic characteristic of generator variables provide operators more comprehensive operating information and they are the base of the probabilistic transient stability analysis in multi-operating conditions. To some extent， expectations and covariances can describe the probabilistic distribution characteristics of these variables. Based on the operating curves of nodal injections and PV voltages and with the assumption that all generator variables obey the normal distribution， the expectation expressions and linear covariance expressions of generator variables are deduced according to the probabilistic load flow analysis and the basic property of numerical characteristics of random variables. In the end， the studies are carried out on an eight-machine system.