In this paper, the very rapid thermal cycles imposed on an ultrafine grain steel plate were successfully simulated using a Gleeble™ simulator. The simulated heat affected zone was obtained suitable for Vickers hardness and impact toughness examination of laser weld that originally bear a narrow zone with high hardness gradient. Effect of preheat treatment on the mechanical behavior of laser HAZ was estimated. It was indicated that the simulated HAZ was composed of fine martensitic ferrite laths with average width in 65nm and 175nm in the case of preheat at 200°C and not, respectively. The laths were cramped by retained austenite film within the prior austenite grain with average size of 9.5 and 6.5μm, much less than that of base metal. The impact toughness of HAZ was improved by laser thermal cycle comparing to that of base metal, and there is no obvious brittle-ductile transition under temperatures from -80°C to 20°C. Preheat treatment of steel plate prior to laser welding resulted in an effective reduction of the maximum hardness and alleviation of the hardness gradient in the weld.