The effect of austenitizing temperature on the microstructure evolution and low temperature toughness of high strength low alloy (HSLA) steel was investigated by OM and SEM. The results show that with the increase of austenitizing temperature from 850℃ to 950℃ while heat treated for 30 min, the average austenite grain size increases from 7.22 μm to 17.39 μm (the temperature of A_C3 is 819℃). After quenching at 850~950℃, the microstructure is lath martensite. The yield strength and tensile strength decreased respectively, and there was no obvious variation in elongation. However, the toughness decreased significantly from 97 J to 31 J. The crystallographic analysis results by EBSD and ARPGE software show that the grain size increased and the variants selection enhanced with the increase of quenching temperature, which show that austenite grain is mainly occupied by a single pair of variants. In addition, the combination mode of the variants for the 850A sample tends to show a CP (Close packed) combination mode. When the austenitizing temperature increased to 950℃, the combination mode of the variants is more likely to be Bain group combination, and the proportion of operation factors representing high angle misorientation decreases, which leads to the decrease of high angle grain boundary density, and the ability to hinder crack propagation is reduced, further deteriorating the impact toughness.