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To improve the laser deposition repair quality of BT20 titanium alloy, a magnetic field device was designed and introduced to the laser repair system. The effects of magnetic field intensity on the macro morphologies, microstructures and micro-hardness of the laser repaired zone were studied. The results show that there is a dense metallurgical bond between the repaired zone and the substrate under the magnetic field, and flat surface can be gained with few powders. And also, the repaired zone is composed of prior β grain with α/β lamellae in it. With the increasing of magnetic field, the α lamellae length declines and its thickness is 0.4~0.5 μm. The micro-hardness of the repaired zone improves with the increasing of magnetic field intensity, but it tends to be homogenization in certain parameters. It indicates that rotating magnetic field reduces enrichment of solute, and then decreases β→α phase transformation driving force. Hence, α lamellae precipitation needs high degree of supercooling, which lead to a higher nucleation rate, resulting in finer microstructures and improved mechanical property.