The influence of laser welding speed on microstructure and element diffusion in YG20/45# steel laser welds are investigated using 5 kW fiber laser under the condition of no groove without filler metals. Weld appearance, microstructure and element diffusion at YG20/45# steel interface are analyzed. On the basis, the influence of laser welding speed on the weld appearance with large coefficient of thermal expansion is discussed between dissimilar materials. The results indicate that for 2 mm base materials, YG20/45# steel welded joint with well metallurgy can be obtained, when the following welding parameters are used: laser power P=1.93 kW, laser scanning speed v=2.40 m/min, and defocusing amount - 8 mm; with the increase of welding heat input, WC coarsening occurs at cemented carbide/welds interface accompanied by large cracking sensitivity. The cracks are commonest in the heat affected zone of welded joint, but may also be situated in the weld metal, which deteriorates mechanical properties of welded joints. The analysis results of line scanning indicate that W、Co in cemented carbide and Fe in iron bring about mutual diffusion, making the joint reaching a favorable metallurgical combination.