In this paper,a new and simple method is presented for studying the semi-classical Laser-Lorenz equations with an injected signal-The pump parameter varies slowly with time,thus it induces delayed bifurcation and transient optical bistability-By scale balancing and approximating to the slowly-varying solution by that of the linearilized system, a scaling transition relation and a condition for transient optical bistability are given-The intervals of values of bifurcation and transition,and hysteresis loops of the transient optical bistability are given by numerical analysis-