Hot wire is used as the filler metal of high beam quality and long focal distance fiber laser for narrow gap welding. The austenite stainless steel base metal is molten by laser, and the Ni-based alloy wire is heated up by wire current. Both the stability and efficiency are improved a lot by using the hybrid heating sources. High speed camera is used to observe the movement of filler wire during laser welding and analyze the effect of welding parameters on wire transition. The solidification microstructure and the element distribution of narrow gap laser weld are studied. Consequently, it is found that wire current and filling wire speed are the two key processing parameters for laser hot wire welding quality. The side wall of base metal is bonded well with weld metal when defocused laser spot with a diameter close to the width of narrow gap is used. The laser reflection mainly contributes to the melting of side walls, rather than the laser beam itself. The solidification microstructure shows obvious epitaxial growth, while the dendrites grow vertically to the side wall due to the rapid cooling speed. No macro segregation is found. However, lack of fusion is found in the bottom of narrow gap with a 90° angle. This problem is solved by using U type narrow gap with arc transition in the bottom. The narrow gap laser weld is finally obtained without macro weld defects.