The work aims to study characteristics and mechanism of temperature variation in the process of the galvanometer- based laser quenching (GLQ) with laser on fly by comparing the conventional laser quenching (CLQ). The surface of 45 steel was treated by two quenching methods and the morphology and hardness distribution were compared. Combined with numerical simulation, the characteristics and mechanism of temperature variation by two quenching methods were studied. There was large difference in the cross-sectional morphology of the hardened layers by two quenching methods under the same total energy input, scanning area and scanning time. The thickness and width of the hardened layer of CLQ sample were 753.66 μm and 3787.21 μm respectively. The thickness and width of the hardened layer of the GLQ one were 256.61 μm and 5808.77 μm respectively. Meanwhile, the hardened area of GLQ sample presented "approximating crescent shape with better uniformity" looks, while the other one presented "crescent" looks. From the cross-section hardness, the hardness distribution of the two samples was similar, which was high in the middle body zone (hardness of sample treated by GLQ was 810~900HV and that of CLQ sample was 790~830HV) and low in the edge zones (about 760HV). From the simulated results, it took 1.25 s for the temperature of CLQ sample to change from 508 ℃ to 1063 ℃ at the feature point during the laser irradiation process and the austenite transformation was 1.17 s, while it took 0.628 s for the temperature of the other to change from 500 ℃ to 718 ℃ at the same feature point and the time of peaks temperature above the austenite transformation was 0.38 s and the troughs temperature was kept above the martensite transformation temperature. Along the laser scanning direction, the temperature change rate of GLQ was one order of magnitude higher than that of CLQ, perpendicular to the laser scanning direction. The temperature change rate of GLQ was two orders of magnitude higher than that of CLQ. GLQ has the characteristics of greater temperature change rate, multiple cycles of heating and cooling process, shorter heat treatment time for phase transitions, as well as less heat accumulation. Therefore, it is expected to be used in thin-layer and large-area quenching.