With advantages of low heat input, high feeding efficiency and low penetration ratio, laser hot wire welding is fit for surfacing and joining thick plate. How to obtain the stable wire transfer is the key problem for laser hot wire welding. High speed imaging is used to observe wire transfer behavior under different welding parameters, which are divided into four kinds: drop transfer, fusing transfer, continuous transfer and wire hit transfer. The continuous transfer is the stable wire transfer and it is the assurance of good weld formation. The differences among the four wire transfer kinds show that the different wire fusion positions are decided by the wire heat. Therefore, the control principles to obtain the stable wire transfer are that wire fusion heat is larger than the resistance heat, and smaller than the total of the resistance heat and the conduct heat from the molten pool to the wire. The temperature of the wire outside the molten pool is calculated. It is found that it is beneficial to obtain the stable wire transfer when the wire temperature is close to the wire fusion point.