Continuous laser welding is conducted to investigate the effect of heat input (HI) on the microstructure and mechanical properties of 26Cr-3.5Mo ferritic stainless steel. The optical microscope (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and energy dispersive spectrometer (EDS) are used to observe the microstructure and the precipitates in the weld. And the mechanical properties are tested by the microhardness tester and the tensile tester. The result show that there are fine equiaxed grains at the center of the weld, and with the increase of the laser welding heat input, the width of equiaxed crystal zone increases, but the proportion of equiaxed crystal zone to the weld cross section increases firstly and then drops. The heat affected zone (HAZ) is narrow. Its width is 0.1~0.2 mm, and no obvious grain coarsening appeares in the HAZ. The HAZ is soften and its microhardness is lower than that of the weld and the base metal. The tensile strength of the welded joint is close to that of the base metal. But the percentage elongation of the welded joint after fracture is less than that of the base metal and decreases with the increase of the laser welding heat input.