A direct method considering transient voltage dip is proposed to calculate the critical clearing time（CCT） ensuring that voltages of post fault power system are higher than limit. According to the stability region theory of constrained dynamic system， the restricted stability boundary is composed of stable manifolds of unstable equilibrium points， periodic orbits and semi-saddles on the boundary， and portions of the boundary of the feasible region. The stable manifolds of semi-saddles are significant part of the restricted stability boundary and they can be approximated by equal energy surface of semi-saddles. The restriction of transient voltage dip is a constraint on post fault power system. Based on the structure preserving model of power system， a semi-saddle point relevant to the fault is found. The intersection point of fault-on trajectory and feasible region boundary becomes a semi-saddle when the angular speeds of generators are set to zero. The critical clearing time satisfying the constraint of transient voltage dip can be determined using the energy of this point as critical energy. Iteration is used to improve the accuracy. The validity of the method proposed is verified by simulation.