Using density functional theory and non-equilibrium Green's function method, we investigated the magnetic and transport properties of small organic titanium-benzene sandwich clusters TinBzn+1 (n = 1-3). The results show that TiBz2 is nonmagnetic while Ti2Bz3 and Ti3Bz4 are ferromagnetic, and our prediction is in agreement with experimental observation. The double exchange mechanism plays a key role in the ferromagnetism of larger clusters. With Ni as the two electrodes, significant spin-filter efficiency (SFE) and giant magnetoresistance (GMR) were found in the TinBzn+1 molecular junction. These transport properties could be controlled by cluster size, bias voltage or gate voltage. Specially, a sign-reversible GMR effect was observed in the Ti2Bz3 molecular junction. Finally, the microscopic mechanisms of SFE and GMR were suggested.