In order to launch objects into low-earth circular orbit using an earth-based laser and a lightcraft with 100kg initial mass, the relationships among the concerned parameters such as the laser average power, the characteristics of laser propagation in atmosphere, light receiver size of lightcraft，the specific impulse of engine, and the energy conversion efficiency of engine were investigated. Meanwhile, the optimal trajectory for such a task was also studied. The necessary average laser power was estimated from the initial acceleration of the lightcraft. The minimum size of the lightcraft's light receiver was derived from laser range. Based on the flight dynamic model of lightcraft and the estimated parameters, the optimum trajectory from the ground to low-earth orbit was computed. The results show that the maximum mass-to-orbit is up to 35.5kg, and the optimum specific impulse is 8140m/s.