In this article, the simultaneous set of nonlinear equations for photon transport and inverted population in active medium is solved by taking the pumping effect into account. The analytic expressions of the space-time distribution function of inverted population and photon density are then obtained for an arbitrary initial distribution of inverted population. The cases of a single or a series of square pulse input are discussed in detail. In addition, the effects of incidence of very intense beam on steady-state operation of the laser are also studied. Under certain conditions, the perturbation method may be used to derive the formulas for photon density and inverted population density in laser. Calculations are made, with ruby laser as an example, and it is shown that the incidence of very intense beam may possibly be used for strengthening giant pulses.