AlMgB thin films were deposited on silicon (100) substrate using a three-target magnetron sputtering system in argon atmosphere. The influence of negative bias voltage on the thickness, morphology, microstructure, local bonding and hardness of the deposited films was investigated. Experimental results show that all films are X-ray amorphous, and the properties of the deposited films have a strong dependence on the applied substrate's negative bias voltage. Deposited at high negative bias voltage, the AlMgB thin films are found to be generally dense, having a smooth surface and containing more well-formed B12 icosahedra, which consequently increase the hardness of the deposited films. However, deposited at low negative bias voltage, the AlMgB thin films exhibit loose structure, coarse surface and contain few B12 icosahedra. It is shown that the hardness of the dense and smooth AlMgB thin films can reach 2261GPa at the negative bias voltage of 40061V.