Abstract: There are a great amount of space debris discarded in the orbits around earth. This pollution in space caused by people's spaceflight has threatened the space assets seriously. Some of the debris with the diameter ranging from 1 to 10 cm, should be given priority treatment for processing, due to the latent but fatal threat they induced. Laser space debris removal is becoming an absorbing topic in the realm of science. Among the mechanisms presented, space-based pulsed laser is one of the effective method to eliminate these small debris. Due to the limitation of technics, the short-pulsewidth, great-energy, compact-structure and reliable lasers claimed in the plans reported were hard to be achieved. A creative debris-removing method using space-based free running pulsed laser was proposed. The long pulse duration of the laser advantaged the impulse generation was revealed in a numerical analysis. Comparing with nanosecond pulsewidth laser, both in theory and experiment, "hot metal vapor ejection" was proved to be better than "plasma ejection" at impulse increment and energy utilization. Analyzing with a laser-melting-metal model proposed, the diameter of metal drops ejected were calculated less than 60 μm in general, with the laser power density greater than 5×10⁶ W/cm². The result demonstrated that the "micro pieces" accompanying the ejection were unable to cause "secondary threat", but accelerated the debris removing in some degree. The study lays the foundation for the application of space-based free running laser in space debris removal.