In order to enrich the preparation methods of biomedical porous Ti, a technology of selective laser melting is used to prepare porous Ti. The effect of spot diameter/scan space (D/d) and powder on the structure of porous Ti and the formation mechanisms of big holes and small holes are investigated. The results demonstrate that when the experiment is 95%Ti+5%TiH2 (95%, 5% are mass fractions) and D/d=1, porous titanium holes are comprised of big holes and small holes, small holes make part of the big holes connected, to be there-dimensional connectivity structure. Laser beam irradiates preset powder selectively, and the area without irradiated form big holes. Big holes are decided by D/d. Big holes are formed under D/d=1, while under D/d=2, are not. TiH2 is decomposed and produces H2 under the irradiation of laser beam, with the rapid solidification and “Piston Effect”, and some H2 does not have enough time to overflow from weld pool and thus form small holes; the effect of small holes on porosity is not obvious, but the existence of small holes improve the open ratio of the sample dramatically.